Sometimes it's better to say nothing at all, but if you can be tactical and explain why you don't like the photo, without ripping into it, then that's ok. It's when people just say 'I don't like that' without saying why they don't like it that winds me up.
If you are giving honest critique rather than just laying into someone because you think it's a duff picture, there is a difference! If someone has asked for comments or critique, they should be prepared to take the good with the bad.