The service is designed to prohibit downloading of the photos. The image displayed on the web site is very low resolution, and that's obviously done by design. My guess is that they don't even have a high resolution version accessible via the web--the original is probably stored off-line to be used for purchases only.
The easiest thing to do would be to preview the large image on the web, take a screen print of your PC, and then crop the image to remove the rest of your desktop. This is still going to leave you with a picture that will be noticably fuzzy even when printed at a 3.5 x 5" size.
Too bad Disney / Kodak aren't offering a digital download service for a smaller fee.
If you're an AP holder, check your recent Mickey Monitor for a coupon for 50% off on the order of Photopass photos. There is a Photopass card inside the Mickey Monitor. Use the ID number on the back of the card as a coupon code when ordering, and you'll pay half price for all 5x7" prints. We had a cute photo of DW and I taken with Chip and Dale at the
DVC member gathering last December. Getting that 50% off coupon convinced me to buy. It was about $8 with shipping and handling.