User picture comments allow you to include optional text along with your user pictures, which will be displayed on your User Pictures page (http://www.livejournal.com/allpics.bml?user=exampleusername, replacing "exampleusername" with your username) under any keywords listed for each picture. These comments are separate from your keywords and each user picture you upload to your account is able to have a comment assigned to it.
When logged in, you can enter any text you choose, including punctuation such as spaces, commas and apostrophes, into the comment text box at http://www.livejournal.com/editpics.bml. Note that you cannot include HTML in your user picture comments and your comments cannot exceed 120 characters in length.
You can give credit to the creators of your user pictures with user picture comments, as they will not interfere with or limit the keywords you add to your user pictures. You can also use user picture comments to provide a description or small story about your pictures, or information about where you found the pictures.
User picture comments will not enable you to post entries or comments with different user pictures. You must assign keywords to your user pictures before you can post using different user pictures.
What are keywords? How do I use a different user picture?
Bear in mind that this is the first FAQ I've ever written, so it may well be in need of some very major corrections. Comments?