This is in the "Skin" of your site.
The default skin is the Gravity skin, and you'll find it in Portals/_default/Skins.
I would recommend that you copy the Gravity folder, or at least save a backup so that you can restore to the original easily. Rename a copy to Gravity-Me or something like that. Then choose the Gravity-Me skin as your site's skin. Any changes that you make will only apply to your site or to others that use the modified skin.
Look in the skin.css file and you will see this:
/* Content Pane - Footer */
#footer {
clear: both;
height: 230px;
margin-top: 70px;
background: url('images/globe.png') no-repeat;
}
Remove the last line to remove the background image.