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GD, imagemajick and netpbm are all used to generate thumbnails of the images you upload. You can use any of them, but it sounds like your preference is set as GD and either your host doesn't have it or the path to find it is wrong.
Imagemajick and Netpbm generate better thumbnails than GD anyway, so try and set one of those up as an alternative if you can. You might need to contact your host to find out if they have it, and what the path is (there was a post about that in here recently).
As far as I know there is no manual for XCGal.
