I was looking at the GIMP site and saw that there are also some tutorials, many of which photo related, and I just discovered that it actually has the CMYK mode... look at this
http://www.gimp.org/tutorials/Smart_Sharpening/
Ciao, Danilo.
p.s.: you may find useful the tutorials you can find there, even if you aren't going to use the GIMP, as, in the end, tools are similiar from program to program.
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