This is just a basic guide to help you critique images. A quick search on the web will give you more detailed, formal guidelines.
Critiquing not only helps the person who's picture is being critiqued, it helps you produce better pictures. Please don't just say that you like a picture, it may make the photographer feel good but it doesn't help them take a better picture.
These are not hard and fast rules but just suggestions to bear in mind when critiquing images.
Form and Narrative
This is how the image speaks to you. Is it just about shapes and lines or is it telling a story. What is the subject of the photograph? Where does your eye start and stop when viewing the photo? If there are multiple subjects, does that make the photograph more interesting or do they create a distraction? Did the photographer follow compositional conventions like using the rule of thirds? If not, were the rules broken successfully? Do the elements like contrast, leading lines and diagonals take the eye around the whole image, or is there wasted space?
Is selective focus or depth of field used in the photograph to blur the background, is it effective? Does the background work well with the subject of the photograph?
Are the elements that need to be in focus, in focus, is the focus sharp? Do the areas that are in an out of focus work together, should more, or less be in focus?
Attention to Detail
Are there dust spots? Is the horizon tilted? Are there elements around the edges of the framerthat are cut off or distracting?
Response to the Image
How does it make you feel? Do you understand what the photographer is saying to you?