Here's StatCounter's public data on browser usage. It should be rounder than W3Schools', which is based on their own logs. The people who go to W3Schools, read: developers, are generally not the best cross-section for basing this kind of thing on. You can also break down the StatCounter data by geographical area, if that's any sort of consideration.
The Wikipedia article on browser usage has links to a bunch of other similar large reports you might also check out for general comparison. There's summaries at that URL, but the Source column links out to detailed information for each.
There's also a great page on Wikipedia showing which browsers support PNG.
Ultimately, though, you need to temper anything you see at from these sources with what's actually happening on your site. If a large percentage of your audience is within certain business sectors, for example, IE6 will still be a significant problem.