There are both. in the UIImages (others.artwork) there are several generic bottom bar buttons, but i have yet to find any app that actually uses these. Even when an app has the same image, for some reason they always put it in the skin for the app itself. (at least for all the stock apps, and everyone i have looked at. But im sure there are some 3rd party apps that use those, but for the most part u have to mod those through the actual app, rather than through the UIImages.
There are also some third party apps that use a wierd setup where they can only show a silouett (i cant spell for s***) so the image is grey, and becomes blue when selected. That image that sits behind the grey image (which makes it blue when selected) is called "UITabBarBlueGradient.png" (a UIImage).
If u want the stock images im talking about, or want to see how to theme them, or for any iphone app skin, etc. go here:
http://www.modmyi.com/forums/winterb...verything.html
Its a complete theme of everything "themeable" in WB format, but with all stock images. Nothing is modded, so u can get anything original u want, and see the layout of anything.