Not sure if this is the right place to post this, but I have had a technical issue with posting photos for several years now, and it is very frustrating. I work on a Mac, and I normally want to edit any photos before posting them - cropping, or masking them. But if I do edit, and then post to a website, they invariably look like just a dot pattern, or plain colored field. Meanwhile the same edited photo looks just fine on my Mac! What is strange is that this problem started a few years ago - previously, on the same Mac, everything was fine.

Cross post to the Newbie Board? Providers are often asking for help with their photos (watermarking; blurring; other effects) but I don't remember anyone describing the problem you're having.
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Post what software you are using to edit your photos.
Provide more info, e.g., what file types are you using: jpg, jpeg, png, gif, ...  
What resolution are you using (e.g., it might say low, med, hi; it might be a number: 4k, 8k, etc.; it might be something like 1920 x 1080 or 375 x 667)
and any other tech details.

It sounds like you are viewing a photo on your computer at magnification (zoom) = 1. All of the dots (pixels), let's say 100 x 100, are properly sized and merge to form a normal picture. When you upload it, with the same 100 x 100 resolution, it might be getting enlarged or zoomed. That will spread out the dots so you only see dots and no picture.

Normal, 10 dots [..........] form picture of a line.
Expand, but keep the same 10 dots [.     .     .     .     .     .     .     .     .     .].  
From the uploaded image, maybe it's showing the same "[1 inch] of dots but you only see part of the expanded "picture" [.     .     .     .] because the dots got spread out.
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Since only you have the details of what you're doing try googling (or AI-ing) for help: "My edited photos only show dots. What should I do?" (Add details: "When using Program-X on a Mac, my ...")
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Have you considered switching to a PC?

Would help to know where you are having that issue.

Photo size: originals vary from 1-2 MB to 100s of kB. Sometimes reduced in size via editing program

Editing program: "Preview" the built-in photo viewing and basic editing app in Mac computers

What sites posted to: X (Twitter), SA, SLF, and others

Newbie board: I have been using computers for over 40 years, and Macs since 1986.

Use PC: I don't have any,

If it's this many sites it's a file type issue.

I agree. MACs typically defer to .PNG format for pics.    

 
But most web pages that host pics are much more .JPG friendly.  Have you tried changing the file format to .JPG before uploading?  

 
Using any pic viewer or editor, do a "Save As" function and select .JPG as the format.  Then try with the new file.  

 

Life is good

 
The Cat

There was no "newbie" insult intended but the Newbie Board is where there may be more relevant expertise. Think of it as a "New PROBLEM" issue and not a "New / Naive USER" issue. It is also where there might even be a better chance of getting a "The same thing recently happened to me and here's the fix" reply.

 
Anyway, the more tech detail you can provide, the more likely that someone might be able to figure out a solution.  
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(If you find a solution, let us know what it is.)