Your answer is below, but I'd recommend that you learn a little bit about skins before you dig in. Take a look at these tutorials:
Pay special attention to the tutorials that have to do with skins. There is also the
DotNetNuke Skinning
Toolkit. I think that Lee's series of tutorial about the Red Leaf Skin have achieved some sort of cult status - they really are that good!
After you've familiarized yourself with skins concepts, you can tackle your skin.
The picture may be either in the skin file itself, or in the skin.css file.
First make a backup copy of the skin zip file and protect it.
Open up your zip file that contains the skin and look first in skin.css. See if the image is mentioned there, and then do whatever editing you need to do to remove it. If the image is specified in a background image tag or something like that, just removing those lines in skin.css will fix it.
If it's not there, it's in the skin file. This will be portal.ascx, portal.html, or some other ascx or html file. You'll need to go into that file to remove it.
When you are done replace the edited files in the zip file and reinstall the skin.