Another vote for ultramon...i cannnot go dually without it anymore
Its possible to make them manually, it was my idea that the creator implemented between beta4 and beta5 to change the way it does its wallpapers......you basically create a big picture set out the way your monitors are (ie, allignment issues and such if your running different sized/resolution monitors, or on different places....the easiest way to see how yours is set up is probably to press Prt Scr and paste it into an image editor) and then just use windows to set the wallpaper as normal, but select tile. This is how Ultramon now makes its wallpapers, unlike the HTML generating it did before which had some annoyances.