They're in /system/app/Launcher2.apk/res/drawable-hdpi. I think they're all in there. In order to make Launcher2.apk browseable, you'll have to rename it Launcher2.apk.zip and then extract it. I copied Launcher2.apk.zip to my SD card and then extracted it, personally. You would then want to rename the copy in /system/app back to its original name, of course.
...do you have a file explorer? The one I use is simply called File Manager. I'm not sure how familiar with Android you are but you can find the file I mentioned using it.