(1) First you need to upload the icon you want to use. To do that, venture to Edit User Pictures:

(2) Upload your new icon by using the form at the bottom of the page. Select your icon from your hard drive and then enter keywords and comments. In the comment section, it is preferred that you put credit for your icon. Credit is given by putting the username of the icon maker in the comments section. There are many ways to word credit but the easiest is by using an LJ User tag.

(3) If you want to add comments to an icon that's previously been uploaded, go to the Edit User Pictures page and input text into the keywords and comments sections beside the icon. It doesn't matter how you word it or how you do it, as long as you put the username in there correctly.

If you use the mood theme on LiveJournal, a link back to my icon journal and/or a LJ User Tag will suffice. If you use the mood theme outside of LJ (i.e. GreatestJournal, DeadJournal, etc) then a Hyperlink is encouraged and recommended. You can add the link to your LJ style or in your Userinfo. You can edit your userinfo by going to the Edit Personal Information page.

Crediting in the User Info