Hi there @Pasta12 - sorry to hear about your missing photo.
Can you please clarify your device's OS version and the version of the Dropbox mobile app installed there as well?
Do you happen to see any signs of it in your deleted files page or the events page perhaps? This is the place where all uploads, edits, and deletions are normally logged so it should give you a hint.
Let us know more and we'll take it from there.