Graphic: [icon, banner, header, etc]
Size: [height x width in pixels]
Pictures used: [link to pictures you want used, or a subject you want used]
Text: [text you want on the graphic. include font/size/colour]
Anything Else: [self explanatory]
If you want to add anything or leave something to me, go right ahead.
I generally make several versions of requests so the requester can pick, and I can always change things if needed.