Cal, its simple resizing one of my custom pics and just dragging it into the background which I was able to do with the layer tool on photoshop. I am realistically thinking any paint type program that will let you resize, move, and deal in layers would make this as simple a job as I had of it. From there just pic your font of choice (check out Erik Stormtrooper for some good ones or ask Cal) and put up what you call your custom and you are ready to print.
Cal, have you tried printing one of these off yet on any type of cardstock? I know Nate has excellent experience doing this so I am gonna bug him, I mean pm him, for some pointers. His trading cards are beautifully printed and his style would lend itself to these cards so easily I would think.