Graphic designing is when you take an ordinary image (like a screengrab, or fan art, or both) and edit it using graphics editing programs such as GIMP or Photoshop, or free online editors such as Picnik, Splashup, etc. Anyway these pics serve many purposes, from being used as site banners to being sold online to simply being made for the heck of it. For example, on this site graphic designers tend to focus on making signatures/avatars/gifs for use by other members:
viewforum.php?f=48If you look around this forum you'll see great examples of graphics in peoples' signatures. Technically even my avatar is a graphic because someone manipulated the color of the original screengrab and added text. So as you can see, you don't need fancy graphics making tools to become a graphic designer either

Hope this helped ^^