Photos that have been submitted to POE can be inserted into a thread using the little tree icon in the bottom right of the posting screen. You need to have the picture number though, which you can find on the individual picture's page.
If you want to copy and paste another photo into a thread, one that's not been submitted to POE, you probably need to copy it from a webpage. I don't believe just copying one of your own photos will paste to a thread. That's been my experience anyway.