Well if you are going to comment "nice post" then its better to only show your support with an upvote.

You say that you can't comment anything else to pictures other than "nice post" but if you really like the picture you could come up with a comment of why you like it, the colors, the setting, the place, the angle, there's millions of words you could put together instead of "nice post"... if you are going to say that just to support, i bet most authors would rather only have the upvote.