Hey there,
I just wanted to point out that the "crunching" done on our pngs produces files that have forced alpha channels. Obviously if the image needs transparency, no harm there. However, for things that should have no opacity (such as screenshots), it can end up bloating them more than they need to be.
Also, I found something that compresses better than pngcrush (however, it is MUCH slower). It's called "pngout", it's cross-platform, it's free, but it is not open source. I'm going to be re-compressing all of our screenshots with it because the results are much better (imho). However, I want to make sure that we get someone with IE to check that they view fine there before proceeding. They do load okay in Opera 10, Firefox 3.5, and everything else that I checked.
A comparison of the 0.47 "spiro" screenshot was as follows:
355726 - Version currently on server 349063 - Saved from GIMP with the unnecessary alpha channel removed 337597 - Compressed w/ pngcrush (brute option) 327638 - Compressed w/ pngout (RGB color option)
28k is 28k... better for serving and receiving. In the end, doing this to all of our screenshots will probably add up to quite a bit.
If anyone is interested in using pngout, it can be obtained here: http://advsys.net/ken/utils.htm
Note: Someone else ported it to Linux, OSX, and BSD... so you'll need to follow a link on that page to the next place.
Cheers, Josh