Experios supports the uploading of CMYK images to your publications. The images however are converted after upload, to their RGB counterparts for better web compatibility.
It’s common however to have some differences in the resulting image. This can result from the ICC profile of the image, not being included in the file. This can be included when exporting from photoshop using the option highlighted below. Note that the name of the profile will likely be different as you may have targeted a different paper or display type.
If the profile is not included Experios will attempt to use a common ICC profile to read the image, but the results might vary heavily depending on the original profile used during the image creation.