It's primarily a manual process.
I just approved the banner; the icon looked about right to me, but if it's 2 pixels too wide, I could believe that. Sorry about that.
On its own, it looks fine. Next to the others, it feels a little large, though. If it'd be possible to make the game pre-filter out images that don't fit the size criteria (too small is no better than too big either), it'd help you out. 'cause yea, I could see how, on its own, 26x26 looks about right.