When updating you icon, you will find these spaces, they are Keyword and Comment.
In the Keyword space you can "name" your icon, so then, when accessing the drop down menu when posting, you can find it by the keywords you gave.
In the Comment space is where you will gave the credit.
You will write this: by <*lj user="username"> @ <*lj comm="secret_graphix">
You need to take out the stars (*) and replace 'username' with the username of the person who made your icon.
Well it should look like this:
Any concerns you can comment with your question here.