There are a lot of services which helps you to optimize your images. But using them is the pain in the ass. First, you have to upload them and then you have to rename them, etc. Here is a quick solution. Smusher a gem for it :)
Once you have gem installed. Now a simple command to compress all your assets ;)
This will reprocess all the images in that directory using Yahoo Smushit service, and then you are good to go :).
PS: There are some options you can pass while compressing the images. For that checkout the documentation of the gem, pretty beautiful and bright docs :).