With banner, blends, colour bars, sigs..etc are simple, the coding will be posted with the graphics.
Here is an example, of one of our promos in my userinfo.
Note the mkdesigns little link? That should be there(if not on LJ then a link when clicked).
When your uploading a icon, the keywords&comments are what we're looking for, well actually just the comments.
In the comment area it should say, Made by, <lj users="orangeinsanity">
If your using a BB forum then this is the coding you should use on your profile/signature:
Avatar made by [url=http://community.livejournal.com/mkdesigns/tag/fee]Fee[/url].
Now photo retouches&manipulations is more tricker because it depends what you use them for. If they are being shown off in a website/post, a simple link to the communtiy&artist in a credits/resource..etc page will do.
With layouts+user info designs, a link to the community (<lj users="mkdesign">.)in your user info will be great.
When crediting us for an icon base you can simple just put out link (<lj user="mkdesigns">) in a resource post, or the place where you keep your credits to textures/brushes/bases you have used.
If using a screen cap, just provide a link back to the community/page you got the caps from.
If there are any other questions, don't hesitate to ask, thanks.