I've tried the program, Liquid Resize, and it is really cool stuff. However, when retargeting to only 75% with original jpeg's in the 6-7mb range (3872x2592), count on a very long time to process. I'm using it on a new machine I just built (64-bit dual-core 3.7gHz AMD, 4gb RAM, 768mb video card) and it's really really slow. In fact, too long for me to even let it finish. A 2mb file of the same resolution processes in about 12 minutes roughly. But, when resizing one, say 400kb (1024x768) , it only takes about 15 seconds.
Again, really cool stuff and I can't wait to see it become more mainstream.