If an icon-maker is ragging on you about "crediting them" & you don't know WHAT THE HELL they're talking about, then this is probably a good reference for your silly head!
HOW TO CREDIT!
Most icon makers will ask you to "comment & credit". Normally comments are so the icon-maker knows who & what people are taking. Commenting is a good way to let the maker know what styles & icons are most popular, so please comment & let us know what you're taking, it's very helpful!
Crediting is mostly done to give respect the artist who made them; as well as redirect OTHER people who like the icon to the icon maker's journal. In order to credit, go under "Manage -> Userpics". This will display your icons:
Type any description you'd like in the keywords box; & in the comments box, type the username of the icon-maker. (If you wanted to credit me for example; you would type _shanghai like you see it above, so it forms a link.)
If you've done it correctly, it will look like this in your userpics page:
Simple as that! :]