Really? omg... I just googled it and it seems to be true :O
I just found this forum question on google: http://newsgroup.xnview.com/viewtopic.php?t=4101
Just another guy confirming, that compression level 6 would be the best.
Oh, and I always thought such compression levels only apply on gimp...
And finally, if your artists are using Photoshop it looks like photoshop automatically saves png's with compression level 6 by default, so my suggestion wouldn't help you...
How cheeky! I might consider changing, then. Is there any reason I shouldn't just use 9 compression?
Greater compression may lead to smaller file size, but also much greater load times.
Before changing the compression strength, you should be thinking about other more significant parameters: indexed colours, bit depth, transparency, etc.http://www.smashingmagazine.com/2009/07/15/clever-png-optimization-techniques/
Here's a couple handy tools for optimizing PNG/JPG/etc:http://imageoptim.com/http://trimage.org/
Unfortunately, these tools may "over-compress" images -- meaning the file size is small, but the loading (decompression) time is longer than usual. Because of this, you might be better off saving the images yourself, choosing the right parameters and compression settings with Gimp/Photoshop.