The way I look at it is, don't just say what's wrong with a pic. Offer actual suggestions on how to improve it. And if your criticism is already said by someone else - either offer a different possibility on how to correct it, or say "I agree with so-and-so." THEN, you mention what you liked about the image. Tell them what they need to improve first, then tell them what they did right.

Remember, kids! Napalm is Nature's Toothpaste!