Masque doesn't
just work with Bartender. It also skins Dominos, Macaroon, and most other action bar addons, buff addons, and any other icon-based addons.
Also, you'd create custom graphics for kgPanels (or anything else) in Photoshop,
GIMP,
Paint.NET, or some other image-editing program. You can create them from scratch, or use other images as a starting point. If you're new to image editing, you'll probably want to start with an image that's similar to what you want to end up with, and modify it, instead of trying to create your own from scratch.
Finally, images in WoW (called "textures") must follow certain rules about their format, color depth, and dimensions. You can find more information about these rules on Wowpedia's
SetTexture API page.