I think all photographers should learn how to critique an image, even if they are critiquing their own photo before showing it to others. So how do you do it? I certainly can't speak with authority, but here are a few ideas...
General Guidelines
Understand the purpose of a critique. Its a detailed analysis of a particular photograph, not the photographer who made it, with the goal of improving the next version of that photograph.
Approach the task as if the purpose of the critique is to help the photographer create a better version of the photograph, not to damage the photographer's reputation, or the techniques (s)he used or decisions (s)he made before pressing the shutter button or during post-processing. Have good intentions.
A critique that merely points out this or that without saying WHY is less valuable than one which explains WHY the suggestions offered are important. Start by deciding what you do and don't like about the photograph, and for each point you're discussing, be sure you know and articulate the WHY relating to those likes and dislikes.
When you make a decision to discuss a certain aspect of an image - let's say - CONTRAST - you should note any personal bias when critiquing the photo. For instance, for black and white images, I personally like there to be high contrast. If I am offering an opinion on a black and white image that seems flat I should first make clear that "for black and white images, I personally prefer contrasty images". This lets everyone involved know my bias and lets them take that into account when they hear my critique.
Remember that a good critique doesn't have to contain negatives. Sometimes a photo is just perfect as it has been presented. There's no need to be critical just for the sake of being critical.
Its also important to be honest, open and frank with your opinions. You don't really help someone when you say that you love an image when secretly you do not. It is better to say nothing at all rather than giving false praise, but I think it's better still to pick out something you honestly like about the photo and then perhaps only one suggestions for improvement.
It is important to honestly tell it like it is, but if you're offering constructive criticism you should be careful to do it in a polite and helpful way. Being rude or nasty doesn't help anyone; it only sows seeds of resentment and causes people to withdraw their future participation.
Before you speak, consider what you like about the image as well as what you don't like about the image. Use the things you like about the image as your starting point when you deliver the critique, then move on to the things you think need improvement.
Specific things you might want to consider when critiquing a photograph
Is the image properly focused?
How is the exposure - dark/light/blown out/blocked up?
Did the photographer make good use of depth-of-field?
Is the image flat or too contrasty?
Does the post-processing add or detract to the photo?
Are the colors accurate?
Is the composition pleasing?
Is the background cluttered or busy?
Are there items in the frame that don't belong or that distract?
Where is your eye drawn to when you first look at the image?
Does the photo have good balance?
Is it easy to determine what the subject of the photograph is?
Does the image connect emotionally?
Is there a clear story line in the photo?
Accepting criticism of your photos
Image critiques from trusted sources can help you improve your photography quickly. Find someone who's photography you respect and ask for their advice. Then make sure to take the final step in this process: Don't get defensive, listen, acknowledge, apply and move on.
If critiques are done with good intention and the person giving the critique has genuine knowledge to share, everyone involved learns something, even the person giving the critique.
