You can just open them in photoshop and save them for web. What do you mean by compressing? Compressing it's size (width&height) or just compressing it's size on disk (bytes).
If you want to save some bytes you can use Yahoo's http://www.smu****.com/ysmush.it/ or you can install pagespeed for chrome and select the optimized images that pagespeed shows to you.
As for the website, it's too colorful, make buttons a solid color (or a softer gradient).