Agree with jettrue's comment on image sizes. I took a closer look at one and it was a whopping 100x the file size that I'd expect to see as a thumbnail on your frontpage.

What you want to do can be done relatively easily with CSS. You've done your work on Simple Zen so stick with that, but look at how the future Zen sidebox borders are constructed (make sure that you look at version 1.2). This approach will work for your more complicated borders too, both sideboxes and center column boxes if you stick to fixed widths (which the nature of your graphics pretty well forces anyway).