Nox makes a good point. There are plenty of photoblogs where an image is posted, but there is no text to tell where it was taken, what the story behind the image is, special circumstances, post-processing techniques, etc. There are others who write a short paragraph about the image, or tell the story behind it, and that always seems to get people to comment with more than just "nice shot", etc.
Not all comments will be constructive criticism - some will tell you that they like it and why they like it, and/or what emotion (if any) it evokes in them. That is still a critique, just not a technical one.
I have to "3rd" Karin's and Mike's comments with regard to onexposure.net. Out of the 6 images I've submitted there, only 2 got published - they only publish the very best images submitted, and it is getting even harder to get published there now. You will get very honest opinions, both positive and negative, about your work - most all with the intention of helping you to improve your skills.