Problem is caused by Adobe Fireworks CS5 as default program for opening PNG files. When I changed default program to Paint they show up again.
I don't think this is really related, I don't have Fireworks and I also have this issue.
Sometime the icons disappear.
It is very random, I can launch C2 and the icons are displayed, and after a while it seems to disappear.
It feels like a random bug, and the workaround is just to over the mouse over the button, their shape is still displayed and the buttons can be clicked.
As mentionned, this is so random, I just "live with it" as apparently there's no way to reproduce it.