No single image embedded in the Instant Proofer can exceed 10mb. Keep in mind that if your image is close to 10mb, the process of embedding the image into the pdf may increase it slightly causing it to hit the limit. So a 9.5mb image may end up being more than 10mb.
It is always recommended to run your images through TinyPNG.com before adding them to the Instant Proofer to ensure this limit is not reached.