tinypng is a utility that will shrink the file size of your png and jpg files.
From their website:
How does it work?
Excellent question! When you upload a PNG (Portable Network Graphics) file, similar colors in your image are combined. This technique is called “quantization”. By reducing the number of colors, 24-bit PNG files can be converted to much smaller 8-bit indexed color images. All unnecessary metadata is stripped too. The result: better PNG files with 100% support for transparency. Have your cake and eat it too!
I tried it with a few png files I had and it does what it says it will do. I didn't notice too much difference in the images and especially in clip art type graphics no difference.
Give it a try.