the icons in the Prototype directory are intended as a
'icon-pool' dedicated for developers, e.g. for solving
Bug 759904

All icons in there a not optimized (e.g. not pixel-perfect),
the are available as vectorial too.

Once they are in use by GIMP they will get all needed optimizations
(Pixel-Perfect, Size.... what is necessary)

Available icons are for symbolical and color icons themes and have 16x16  pixels.