I suspect that people post pictures by different methods. In the old days of internet forums, people generally used a picture hosting site (like photobucket). Store your pictures there. Then use a somewhat convoluted set of steps to link to the picture when you post.
For this forum that currently runs on the Xenforo platform, there is a button ("attach files" with a paper clip symbol) just below the window that you type your text into. Easy to use and I believe it uploads a copy of the picture from your computer or phone directly to the forum server, where I think it resides forever to reappear along with your post (or until the forum owner deletes it). There are many old threads on the forum that no longer display the pictures. Because they are no longer stored at that photobucket or similar site. Maybe the site no longer exists, maybe it was a paid site that the poster has dropped out of, maybe the poster deleted the picture from that site.
I think the "attach file" button here solves those issues. It accepts most photo formats/extensions, but not all. I think it does not accept a .png format, for instance.