the xmb_ingame.rco images are noticeably lower quality than the ones used in the actual xmb by the way.
there's also the fact that editing icons of ingameXMB seems to corrupt them all aside user and settings icons, which is not only weird but also indicates the fact that
ingame xmb seems to have a very limited amount of resources on it's use...

i had zinga look at them earlier when i was building up the first version of my clock and what comes outside of things, the gim's don't seem to differ from normal ones at all.
they just quite simply don't work, whether they're smaller, same size or larger than the originals.

only way to rebuild ingame XMB rco and have all the category icons work, is to not convert them to png and back. You can edit the other images, but if the main icons are converted they always break.