It would be nice to have some form validation on new posts, specifically on the "picture URL" section, which seems to be a main source of posting errors for ads, often being used for a link instead of an image URL.
 
Simple regex can fix that, by making sure that the image URL provided ends in a valid image file extension (jpeg, png, webp, etc).  

Hay! There is no longer a "chain link" icon to indicate a hot link within.
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I've been posting stuff like this:
"MINIMALLY, a "picture link within" icon"
Posted by  impposter  at 5/22/2017 4:32:53 PM (S&P Board)
for a long time (but that post expired or no longer available).
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Here's one that is still available:  
http://www.theeroticreview.com/discussion-boards/newbie---faq-33/there-is-a-link-inside-that-post-170503
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When there is a link in the "Link URL" [ box ], the subject line has a chain link icon appended. (It should be there on this post.)
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When there is a link to an image file in the "Picture URL" box (like this):
[ https://cdn3.iconfinder.com/data/icons/glypho-generic-icons/64/link-chain-1024.png ]
there is NO icon or indication on the subject line that there a photo / image / picture link within.
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Bandwidth and speed are less important than they used to be, but I didn't want to waste time opening an AD post with no photo within. Nor did I want to waste time opening a Photo Only post without a Photo!  
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What I have found is that TER uses a very primitive text check on the last few characters (extension) of anything placed in the "Picture URL" [ box ]. The last characters MUST be .gif, .png, .jpg, .jpeg (and maybe one or two more). I don't think that .svg (vector) files are allowed. TER does NOT check the complete link for validity.  
http://cdn3.iconfinder.com/data/icons/glypho-generic-icons/64/link-chain-1024.png is (currently) a valid link AND it is acceptable to the TER checker / validator.
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http://cdn3.iconfinder.com/data/icons/glypho-generic-icons/64/link-chain-1024.png&w=800&h=600 might be a valid link but it ends with a size delimiter (&w=800&h=600) and TER would reject it outright with an error message. Something like "... must be a valid image file."
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This_is_NOT_a_valid_link_to_anything.png is NOT a valid link under any circumstance but I think that it would pass the TER link checker because it ends in .png .
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Many errors with the loading of image URLs fall into a few categories.  
(a) VPNs being blocked by the image hosting site. All you see is a big empty box in the TER message. The VPN allows the TER frames, text, and message to get through, but not the image itself (which bypasses TER).
(b) Submitters not removing size or other info appended a valid picture extension.
(c) Images being deleted or moved from their original (once valid) URL location. ("Image no longer available." or "Image not found." and so on.)
(d) Providers exceeding their allocation of FREE monthly bandwidth on some image hosting sites. ("You have exceeded your monthly allocation. Pay for more bandwidth at [hosting site].")  
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I'm not sure, but I think your "regex" suggestion is just to search for "regular expressions" (pieces of text) and TER already does that. The problem is, I think it ONLY checks for the final letters or period.letters and can be fooled by bad links with valid URL limiting letters appended thereto. I'm not sure, but TER might also regex check for a valid http or https at the beginning.  
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Exactly what errors are you noticing? Are any of them explained above?  

Oops! Yes, there is. Orange box with chain link inside.

A simple “preview post” button, which I’ve seen on other forums.  

For me, it's the usual number of "blank images." But I've been using a VPN for a long time. Sometimes, I will cut and paste the link directly into a browser tab (no TER involvement at all) and the image is still blocked, sometimes with an error message: "Access denied" (you are using a suspicious IP address = the VPN IP address), "406 Not Acceptable," "The image "https://[actual link]" cannot be displayed because it contains errors." BUT when I switch to a different VPN, the image DOES load correctly -- no errors! -- , ... and several other error messages that I can't reproduce at the moment.
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To try it yourself ... hover your mouse over the "empty box" where you are hoping the image to appear. Right click and choose "Copy image url." Open a new tab in the same or a different browser. Paste the URL, hit return, and see if it loads correctly. If it does NOT load, the fault is not with TER. (Once in a while, you will see that a Provider has tried to post a pic from their phone or computer, NOT an image hosting site: "http://C:\Users\JaneDoe\Pictures\...\red_panties.jpg"). "C:\Users ..." passes the TER sniff test but it is clearly a non-accessible personal computer.  
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I use a VPN that constantly changes the actual server being used. If I re-load (refresh) my TER browser, a "broken" image link now loads fine because the image hosting site now allows their content to be delivered to that IP address (my browser).  
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Because of the VPN variability, EVEN IF someone (a poster) was successfully able to "Preview Post" and see the image appear in their own TER preview post, it might not work for someone else using a different VPN or even for their own viewing if they are using a VPN that keeps changing.

Here's an example ad that has this issue:
https://www.theeroticreview.com/discussion-boards/san-diego-ads-95/first-time-in-sd-february-21st---24th-date-pretty-victoria-lace-24700?page=1

I'm no network guru either. But you've picked a good example.  The fact that the post went up with this [ https://x.com/VikkyLoveLace/status/jpg ] in the Picture URL [ box ] makes me think that the TER validator deosn't even check for period-jpg ".jpg" , but just "jpg" as the last three characters (also allowed: jpeg, gif, bmp (bitmap), and maybe one or two others).  
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Knowing that the Provider has images here: https://x.com/VikkyLoveLace/status/jpg , it is SORT OF easy to edit it down to https://x.com/VikkyLoveLace/ which is her X home page. Now just scroll and click for photos.  
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That is definitely a poster-error inserting a nonsense URL that TER did not detect. In my case, it is also, partially, a VPN problem. Every time I click on a link to X, my browser warns me:
   Warning: Potential Security Risk Ahead
   Your Browser detected an issue and did not continue to twitter3e4tixl4xyajtrzo62zg5vztmjuricljdp2c5kshju4avyoid.onion.  
   The website is either misconfigured or your computer clock is set to the wrong time.
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There is NO WAY I am going to change security or other settings on my VPN browser just to see some photos of "what I had for lunch today" or stories about "I went to Starbucks today and got a large coffee for the price of a medium! Whooo!!!" So, I mostly don't bother with X (even in real life).
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Proactively, you can PM Vikky LoveLace and tell her to ask for help on the Newbie board about posting pics. (If no PM, post a reply to her San Diego Ad telling her about the error, etc..)

Good info. Thank you.