JJ, I'm not a techie, but let me explain as best I can....
The viewer can't use a regular photo to render clouds. These files (see the downloads graciously posted above) are a bit like sculpt maps, RAW files, or alpha masks. While they are tga format, they are not photos that are simply rezzed in the sky as a texture might be rezzed on a prim. Rather, they hold information for the viewer to help it render--make--the clouds you see. These files determine the relative shape of the clouds; other settings (found in your environment settings) determine things like density and repeat of the clouds across the sky.
If you want a picture of the sky from your back yard rezzed in world, at this time the only way I know how that can be done is to put that picture on a prim or prims. [As an aside, a webcam streaming sky might be a cool thing to do on media in world.]
So far as your viewer crashing, I'll also bring up that mine did, too, even with these files, until I decreased the file size to 512x512. I'm in Kirsten's; currently 21 (5), but have been in 21 (6) as well. Try downloading one of the files above, resizing it, and see whether you get better results.
And, if a more tech-minded person can elaborate or correct me, please do!