I just made my first wallpaper for Slackware. I'm not a graphic artist by any stretch, but I'm pleased with how it turned out.
I've attached small preview copies of the two versions (with Tux/without Tux), but I have higher res versions located at http://donttakemypicture.org/slackbg/
I've made 5:4, 4:3, 16:10, and 16:9 versions, with resolutions of 2560x2048, 2560x1920, 2560x1600, and 2560x1440.
Most people (myself included) don't have screens that large, but I figure it's better/easier to scale it down than up.