Problem: You get an error that states that CMYK files are not support.
Explanation: Image files can be made in different ways depending on it's intended use.
Images using the CMYK colourspace are intended for print, CMYK stands for cyan, magenta, yellow & key (black), these are the colours used by printers.
Digital screens however use RGB (red, green & blue).
Solutions:
1. The best solution is to speak with the person who created the content for you and have them produce an RGB version.
This will be the best bet to ensure the colours look the way they were intended.
2. If this is not possible, you may consider using an online tool to do the conversion.
It's a good idea to check the output to make sure it looks the way you expect, depending on various factors the conversion can change the way the colours look.
Disclaimer
Aeris Solutions does not endorse the use of any of these particular tools, nor are we able to support these systems.
Please evaluate these tools and consider their suitability for your purposes.
Aeris Solutions has no affiliation with any of these websites / tools and offers no guarantees or assurances.