Could you tell me the command line you're using ?
I think you may have a problem concerning the compression of .sea files
On both Mac and windows, I use "the unarchiver" to uncompress the .sea.bin images from apple, then, for coherence sake, I rename the resulting ".sea" files to ".img" (to avoid mixing-up with old compressed mac executables)
NB : usually on windows, the last extension is hidden (.sea.bin becoming .sea). don't let this mechanism fool you about what kind of file you have.
On a side note, about prepared images, the ones on the wiki are only empty hard drives, if you are looking for preinstalled mac os drives, you'll have to look elsewhere.