The only reliable way to post a pic is to have it already available on the internet and link to it.
Upload it somewhere (or find it already existing), Get its address, and paste it in there, viola, a pic. (Ignore the big red X...that was for something else.)
There is a technical barrier to this that Those Who Run This Place have not been able to resolve. So, for now and until something fundamental changes, the answer has to be this:
Some appear to be able to post pictures, while others cannot.
I know that's not particularly satisfying, but it's at least clear. Just pretend we're in a fantasy world and this is one of the bits of worldbuilding.