Last night I did a presentation with Russ Tokars on post processing images. Russ tackled Photoshop while I used Lightroom. During the presentation a member made a good mention about how Flickr.com is a great way to have access to the works of many photographers. It allows you to browse through for inspiration and ideas on not only composition but on post processing as well. During that conversation I mentioned that Flickr also has many photo critiquing forum groups where a member can post an image and the Flickr community offers constructive criticism. I was asked to post some of the groups but (as you will see) there are way too many to list. Instead I will offer some suggestions on how to find one that fits your needs.
First I have to restate something I mentioned at the presentation. Having someone critique your work can be a hard blow to the ego. If you are overly sensitive about your work you may want to reconsider. However, keep in mind that the people making these observation don't know you and could care less about your feelings (without being rude). They react to a request and will offer an opinion based on their level of experience and expertise. DO NOT take anything that is said about your work to heart. Instead work on separating yourself from the image and trying to look at it from their perspective. After all, they are looking at your work with fresh unbiased eyes.
So how do you go about finding a group? If you click on the 'Groups' tab on the top menu it will open a drop down menu. Click on the 'Search for a Group' link and it will take you to a new page. In the search dialog box make sure the 'All Groups' is selected and type in "critique" into the dialog box. A new page will open with a long list of groups (2,949 as of this morning) so you see why I can't offer a list. This is where some work comes into play.
Start going down the list reading the intro description. This is your first introduction into a group. Just by reading these few lines will give you a feel for the group. Click and open groups you are interested in into new pages. Go ahead and continue browsing the main list until you have a few groups opened. Now you have to see if the group is what you're looking for. Read the group's rules and start browsing some of the most recent posts. Look at the poster's image and start reading some of the critiques. Is the information being offered legitimate? Is it constructive? Remember you want a group that will help you advance. You will not advance if all the posts are, "nice photo. I love the colors." Or, "I like the composition." You will not learn anything from this type of critiquing. These are what I call 'ego boosters', people being nice without adding anything intelligent.
Bookmark the groups that capture your attention. Continue through the list. "But there's over 2,000 groups! How can I go through all those?" You don't have to. Within a short time you will find a few groups that you like. However, I do suggest that occasionally you visit some of those groups further down in the list. You will find there are some gems hidden in there. In particular, there are critique groups that are subject specific. If you're a nature photographer there are groups for that. That way you won't be reading critiques on, let say, portraits. If you're into portraiture, find yourself a critique group that targets that type of photography. If you're all over the place, there are general groups galore.
Whichever group(s) you decide to join, the key is finding members that offer legitimate, well described and objective criticism. Most all of these groups are also set up that participation is two ways. Receive a critique, offer a critique. Not only will you learn from what others see in your images, you will also learn by analyzing someone else's work too. The benefits are many. Now a word of warning; Critiques are given in a written format. As you know, intentions are not transmitted easily through the written word and often what you write can be easily misinterpreted. Mind your grammar and choose your wording carefully. You wouldn't want to inadvertently piss someone off by what you write. You also don't want to get banned because of a comment you post. With that in mind, remember that others may have the same problem and what they write may come across in a way they did not intend it to be. Take what is written with a grain of salt. You don't have to agree with everything people write, it is a learning tool only.