We have been going over this for a while now and I have written a helpful posting over in the Information Systems blog.

However, I did come across this today and it got me looking on wikipedia and on Google.

Let me give you a comparison

JPG – 9.33KB

PNG – 48.1 KB

GIF – 18.4 KB

I have turned the compression up on the JPG to show that it uses lossy compression.  Download the images and use an image editor like irfanview to zoom into the pictures to check their quality.