You’re right, my link is broken. Google for images that are 4096 x 1024 in size, and you’ll get 18 different possible skydomes (at time of this comment). An even better solution is to make your own skydome from a panoramic photo (perhaps one you’ve taken yourself and stitched together with Hugin). 4096 x 1024 seems to be the ideal size, and keep in mind that about 200 pixels of the image will be cropped from the top and bottom when it’s displayed by Compiz.

I couldn’t tell you why the sphere doesn’t work. You did change the number of virtual desktops to 4 right? Maybe you should look at this more updated post: Customize Your Fresh Ubuntu 10.10 Maverick Meerkat Installation. Even if you’re using 10.04, the Compiz settings are the same I believe.