This would improve the package size, the interface loading speed, solve problems PNG has with transparency support under IE, etc. and GIF is now patent free.
After experimentations, it seems we don't gain much by converting all PNG into GIF. ALso we tried to apply the pngfix.js hack for IE < 7 but it doesn't work in Piwik.
Therefore we will stick with our PNG and hope people will use Piwik with IE7 or mozilla or opera or safari :) (it works with IE < 7 but icons are less beautiful)