You just click the “browse” button at the bottom, in the attachments section, and select your photo. What happens when you click “browse” depends on your phone a bit. On mine, a samsung android, I see recent files and can select which app to look through so I pick “Photos” which is the google photos app to browse my camera pictures. I’m not sure what it looks like exactly on an iphone. If you want more pictures click “add another file” and do it again. If they are full size pictures you can probably only attach 2, maybe 3, at a time to stay under the file size limit. The server is also really slow so your post will show up without pictures at first but they’ll display eventually.