I don't have a solution for stopping this when "uploading photos" but I found that Dropbox doesn't rename my pictures if I say to upload file instead. What I mean is that on my Android app when I click on the "Add to Dropbox" icon it gives me a menu of options.
Two of those options are:
* Upload photos or videos
* Create or upload file
If I choose "upload photos or videos" then Dropbox messes with the filenames, _however_, if I choose "create or upload file" I can just go to my Camera folder and select the photos I want to upload and it _does not_ change the filenames.