There's no technical issue , but which device or browser are you using. That could be the issue.
Easier on a laptop / PC and with Chrome, that seems to work best.
Even there it's not an obvious find !
In that message screen to the left where it says send message, is an icon looking like a mountain in a square in an L , is best how to describe where you need to click to add a photo!
eBay don't make some things easy ! 🙄
@strangestash