Ubercart

One way to add content to a sidebar is to create a block. Add an image link in the block, etc. Here is an example that uses an image block in the center column text area. The disadvantage in this case, is that the center column is not fixed width and will vary by browser window size. However, in your case, most sidebars (left/right) are fixed width.

This may not directly answer your question, but it is another consideration that works for some applications.

Jim