I've found that giving a constructive critique is an artform in itself. If I like something about an image, I always try to go beyond the "nice pic" or "excellent work" comment and give a specific reason that I like that particular image. For instance, if its the composition, relate what it is about the composition that is attractive, or, if it's the color or lighting, give a reason that these elements are pleasing. If there is something that troubles you about an image, don't immediately go on the attack. First, try to comment on something that is working well in the image. And with most any image, there is bound to be something that is working well. After first pointing out what you like about the work, only then "suggest" they might want to try this or that, whichever will improve the image in your opinion. Telling someone blunt out "Hey, you "dropped the ball" on this one" will only get you a blunt reply and a angry one at that. But saying "Hey, did you MEAN to "drop the ball" in that way" gives the person a dignified way to respond to your critique. they won't feel so much like they totally screwed up. It's more likely that you won't be putting them in a defensive mode.