This happens in Ambiance as well (see the welcome screen in granite-demo for instance). My guess is that GTK+ tries to style the gray areas of the icons to match the hover/prelight colors (obviously this doesn't happen with icons that were loaded as pixbufs).
Maybe you could fix this from the GTK theme by overriding the general prelight state for GtkImages?
This happens in Ambiance as well (see the welcome screen in granite-demo for instance). My guess is that GTK+ tries to style the gray areas of the icons to match the hover/prelight colors (obviously this doesn't happen with icons that were loaded as pixbufs).
Maybe you could fix this from the GTK theme by overriding the general prelight state for GtkImages?