Can you recommend me one? I'd like basic functionality cropping, rotating, saving.
@Niklas Rtz, for what you describe, you most likely already have installed. If not, it's free anyway.
I just use the Google+ Photo editor that's already on my device, and it has everything you mentioned:
If you don't want to use that particular tool, then check out PicsPlay Pro which isn't free, but very powerful.
More options can be found here: https://alternativeto.net/software/pixlr/?platform=android