There is no need to convert your images to CMYK.
The Print on Demand feature is integrated into the website system. Everything you upload will be RGB images.
CMYK conversion is pertinent to traditional printing processes involving traditional mechanical printing presses.
The high resolution large format inkjet/giclee printing process produces just as high quality and color printing in RGB.
Since the print vendor would be printing from images RGB (or sRGB) that you upload, the quality of the print will be directly commensurate with the quality of your digital image.
If your images are high quality and sharp, it will carry through to the print.
There is always a possibility for some color shifts at times but this will be true of any printing service unless they did the imaging and printing together in a controlled environment while color correcting and proofing against the original.