Free software for CMYK image conversion/editing?

Question

I've got some RGB images I'd like to print, at local printer and on print on demand sites. Apparently I need to convert to CMYK for best results. Is there good free software where I can convert RGB to CMYK and then adjust the saturation, contrast, brightness etc (as CMYK)? Thank you.(So far I only know of Scribus, but seems it can only save as CMYK, not adjust the levels.)

gus_massa · Accepted Answer

I use Gimp, but I think it does not support CMYK.Searching for a while, I found that Krita supports CMYK, but I never used it.