nruddock is right. Only GIFs or XBM-files (a format mostly used on unixoid OS's).
The way out is indeed PIL. It's easy to use if you have found out to handle it.

The following code is untested, but will give an idea:

import Image as PIL
import ImageTk as PILTK

class ImageButton(Button) :
        def __init__(self, master, **kw) :
                img=kw.pop("image",None)
                if img:
                        self.TKimage=ImageTk.PhotoImage(PIL.open(img))
                        kw["image"]=self.TKimage
                else :
                        self.TKimage=None

                
                Button.__init__(self, master, **kw)

The class ImageButton can now be used like a standard button. If you give an imagename, the image is opened via PIL (so nearly any format is supported) and a Tk-compatible image is buffered in class ImageButton (this is needed to avoid that the image is removed via Pythons garbage-collection). The filename is than replaced with a pointer to the image, so the original init-call is done properly.

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