For Dropbox, I'm not sure that there are any apps which can access the files/photos other than the Dropbox App. You might want to search the Play Store for this and see what comes up.
I think you i starting understand what you mean You need dropbox client that identify as gallery app so it can pic pictures for other application. Samsung's Gallery app have that function, you might find other, try searching file menagers with dropbox support. If you able to find such application you will able to "use" pictures in dropbox on any app