Duncan Davidson has recently written about creating high dpi images for Safari on the new iPad (3):

"After reverse engineering the limits and then later being pointed to the documentation about the limits, a birdie dropped a hint into my inbox telling me to try using a progressive JPEG file. Within just a few minutes, several people also chimed in noticing that the large iPad hero image is a progressive JPEG. I’ve not used progressive JPEGs in years, but following the lead of the birdie and the crowd, I gave it a shot. Lo and behold, it’s indeed the ticket to large JPEG joy on the iPad Retina display."

Did you know that Acorn can make progressive JPEG images? Just choose the File ▸ Web Export menu item, choose JPEG for the file format and click on the Progressive button:

And now you can display big JPEG images for Safari on your retina iOS devices!