Summary: If your already saving images for the web out of photoshop using the “save for web” tool ( under the file menu ) then your not likely to get big gains using Smush.it.
I saw a tweet about yahoo’s http://www.smushit.com/ysmush.it/ Smush.it service that is supposed to compress images your using in web design without losing any quality …. I ran it against 40 files and eeked out a measly couple percent at best. All of these images ( jpgs, png8, gifs ) were exported from photoshop cs using the save for web tool.
There has been lots of talk about making your site super fast to download in order to get better search engine rankings. It is not a big factor and it also must be considered that if you make your site super duper fast by not having any images at all then your not likely to win anything cause your site would be plain and the human reaction would be negative. Why not just remove the giant slider on top of the site? Or remove the big background image? Or get rid of jquery?
Ok, all things being equal then compressing your site gives you the edge but I think our time is better spent elsewhere. Like putting something of real value on these websites.