Kraken.io supports sRGB and CMYK. CMYK images are auto-converted to sRGB colorspace images by Kraken.io. We do, however, recommend that you supply only sRBG images by converting your CMYK images yourself, to ensure that the quality is what you expect.
The reason we recommend uploading only sRGB images is that we cannot guarantee that the automatic conversion to sRGB will retain the same saturation of the uploaded CMYK image. We estimate that less than 1% of auto-converted CMYK images might look slightly different (either washed out or over-saturated) based on customer reports received over the past two years.
In any case, CMYK is designed specifically for print images - CMYK refers to the four inks used in some color printing: cyan, magenta, yellow, and key (black).
Do yourself a favour and make sure all of your images are sRBG colorspace ones to begin with!