Please note that these are just examples, since I'm really not that picky I'm happy with you just crediting in some way :)
 Save the icon you've taken to your computer [name it for example: integralby_icongalore]
 In the Navigation go to Manage >>> User Pictures
 Upload the icon to livejournal
 Credit by writing in comments/keywords and save settings.
 Your user-picture page will now look something like this:
 You can also credit in userinfo. A simple "Thank you _icongalore (or hysteriagalore) for my [whatyouarecallingtheicon]icon(s)" will be just fine :)
I assume you already know how to edit your userinfo page so I'm being lazy and won't include pictures of that ;)
Hope it was helpful!