I think for now images that compliment retina displays will only exist in specific apps or photos. Web content will continue to serve compressed images for speed and global compatibility with things like 2g, 3G and slow broadband connections.
Google the gatekeepers place a high relevance on speed, some big advancements in speed profiling in analytical for web masters. Google also factors site speed when ranking web sites.
High res web images for web pages and twitter will require a lot more bandwidth. I think the only time we will see high res web images in the next 5 years is if people target the iPad specifically and deliver a different set of images.
High res ui on web pages will become more common place v soon but web images I think will stay compressed for some time to come.