I always use JPG and it works just fine.. same parameters as usual.. highest image dimensions and 300 ppi. I've printed thousands of prints with them and like I mentioned.. only a couple issues but the image quality has always been top notch. From wallets (die punched for easy removal) all the way to canvas wrapped 20x30 prints... great stuff.

Their books are a bit more expensive but I'm sure are also of high quality.. i'll need to try one of those soon.