http://www.modmyi.com/forums/skinnin...ok-inside.html
some are named icon.png, some are named other stuff..
but they arent like changing regular icons, if you just replace the appstore ones with a new icon, the app wont work anymore.
you have to save the icon file from the appstore app folder, use FIXPNG (google search it) to then view/edit it, then open it open in like photoshop or anything else and paste the new icon of your choice over top of the old one and save it. then replace it like normal.
difficult but its the only work around for the appstore ones cause just replacing them causes the app not to work anymore. you have to modify the original file icon file.