We had our A6 flyers rejected by Print 24 because they weren't in a CMYK colourspace - I'd saved them as .jpg and away we went. Silly me- print24 did give notice about only accepting CMYK, but I thought I'd wing it and let them do the conversion.
Despite Gimp being a fantastic Image creation tool, it has it's limitations. The main drawback of it appears to be that it cannot do CMYK exports.
# apt-get install libtiff4 libtiff4-dev mrwtoppm-gimp liblcms1 liblcms1-dev
# mkdir -p /usr/share/color/icc/
# cd /usr/share/color/icc
# mv */*.icc .
Bodge the libtiff as separate assumes libtiff3
# cd /usr/lib
# ln -s libtiff.so.4 libtiff.so.3
# gunzip separate-gimp2-0.3_linux.tar.gz
# tar xvf separate-gimp2-0.3_linux.tar
# cd separate-0.3_linux/
# cp separate /usr/lib/gimp/2.0/plug-ins/
# cd separate-0.3_linux/
Instructions - lifted from the separate website
Separating an image:
To convert an RGB image to CMYK format, bring up the right-button menu, and go to "Image->" If the plugin in installed correctly, there will be a new menu, "Separate". From this new menu, select "Separate (normal)"; you will be prompted to select an RGB source profile, and a CMYK destination profile.
If you have installed the Adobe and sRGB profiles as per the instructions in the archive, you can just accept the defaults for testing, otherwise you'll have to locate the profiles manually.
A new image will be created with four greyscale layers, named "C", "M", "Y", and "K".
This gives a rough reconstruction of the colours, and is the next best thing to a true CMYK painting mode, since you can paint on the layer masks, and see the results in realtime.
*NEW for 0.3* - the "primary" colours chosen for the "Separate (colour)" mode are now much more akin to the primaries used in printing, which are nowhere near as bright, saturated and downright lurid as pure screen Cyan and Magenta! This gives a far more realistic preview of the colours.
Saving:To save a CMYK TIFF for use in a DTP program, bring up the right-mouse-button menu, and select "Image->Separate->Save...". Please note that saving an image this way won't clear the image's modified flag, so when closing, you'll be warned that the image hasn't been saved. This won't be fixed until the plugin is capable of loading CMYK images. Until then, if you need to edit the CMYK image in the future, you need to save it off in XCF format, to preserve the layers.
I used krita (KDE equivalent/successor to Gimp) to check that the images were ok - it recombines the CMYK and shows you the finished product.
All ok,so I zipped them up and sent them off to print24 - they were accepted!
# apt-get install krita
$ krita A6_flyer_page_1.tiff &