Try making a folder in the "AddOns" folder, putting your graphics in there, and adjusting your file path in kgPanels accordingly. It would look something like this: "\\Interface\\AddOns\\YourArtFolder\\panelart.tga". You don't necessarily have to put it in the kgPanels folder.
Seerah, I don't know when that started, but I still have mine just in a folder in the "AddOns" folder, and they work fine.
Also, make sure your artwork is the correct size. IE: WoW will only use artwork that is sized correctly in powers of 2. 2, 4, 8, 16, 32, 64, 128, 256, 512, and I think the largest you can make an image is 1024 (but I'm not too sure on that one). These measurements are in pixels, and are per side. The image doesn't have to be square, but each side does have to measure in powers of 2.
You might want to have a look at the
kgPanels Wiki.