You don't say where your pic is.  If it's just a pic on your computer, that won't work because no one can access your computer other than you (I hope).  

File type. It has to be a recognized picture file type. I've posted .jpg, .jpeg, .gif and .png files.  If your link ends with .html, .htm, .doc and so on, it's not a picture file and TER software will reject it.

TER doesn't store your pic. It stores the LINK to the pic and just acts as a middle-man.  When a reader opens your post, the TER computer has to find your picture on some other website and ask for it to be temporarily downloaded to the reader's browser so they can see it.  

2. Location and access - If your picture is only on your personal computer, it is not accessible to the outside world. (I'm ignoring private servers and all that stuff.) So you have to put your pics on a public server that allows public access.  Imagecoast and others are picture hosting sites.

Create an account on the host. www.pichost.com/youraccount
Put your pics mypic1.jpg, mypic2.jpg, etc. there.

www.pichost.com/youraccount/mypic1.jpg
www.pichost.com/youraccount/mypic2.jpg, etc.

You put a single link, www.pichost.com/youraccount/mypic1.jpg , in the "picture URL" box at the bottom of your post.

If pichost goes down, your pic won't load.
If you remove your pic, the TER reader will get an error ("File not found" or similar. The LINK is still there in your post, but the TER computer can't find the original file on pichost.)