So, tutorial.
This is what to do to credit icons that are already uploaded, it's pretty much the same for icons you're gonna upload.
You go to Manage > User Pictures, like so:
You, of course, click on it.
You'll get a list of icons with empty fields or not so empty ones in front of the icons. In keywords, or in comments, you write [lj user=nameofthepersonyougottheiconfrom] (replacing the [] by < >) or just the name, without the code. Like so:
You scroll down to the end of the page and click on Save Settings. Now, go to your icons page, and you should see something like this:
And, that's it! You're now crediting the icon maker. See? it's not something hard. It will take you about one minute. Or less. Is one minute that big of a deal when an icon can sometimes take up to one hour to be made? No.
You have no excuse now not to credit icon makers.