How to credit icons
In the keywords, put whatever you want to describe the icon. In my example, it's a Marilyn Monroe icon. So I have described it as such.
In the comment area, put the maker's username as <*lj user="USERNAMEHERE"*>. Take out the stars, replace USERNAMEHERE with the maker's username. In the example above, I have replaced USERNAMEHERE with marilynstar. marilynstar is the maker.
How to credit other types of graphics
Copy and paste <*lj user="USERNAMEHERE"*> into your profile, sidebar or friends only entry. Remove the stars, replace USERNAMEHERE with the maker's username. Header/banner/layout/etc. made by <*lj user="USERNAMEHERE"*>.
-It's good netiquette.
-You acknowledge people for the work they put into making icons/graphics.
-Others know where to find more of a certain maker's work.
-Need more reasons? Read teh_indy's Do-s and Don't-s in the icon community.