"Only JPG files, RGB, at least 3 Megapixels, saved at the maximum quality will be accepted."
Your error message means that either you haven't gone large enough, OR you saved CMYK instead of RGB.
Just experiment with the size of the vector before exporting...until you get it large enough.
It IS created in Illustrator, I only mention opening it in Photoshop because that's the only way you will see how the export/conversion to JPEG looks at 100% which is very important. posted in I so need help