The next release of the GWT Widget Library should be out by Tuesday. The main addition thus far are the ImageButton and ToggleButton widgets. Both of these support the usual JPEG and GIF, as well as PNG.
If you are unfamiliar with PNG it may be because IE6 does not have proper support for it, although there is a workaround. The new button widgets take advantage of the workaround and allow the use of PNG in IE 5.5 and IE6. The advantage of PNG is that it supports an alpha channel, allowing you to see through the image. This avoids having to deal with matting, and the image looks good no matter what background color is used behind it.
Here is a screenshot of an icon bar consisting of several ImageButton widgets using PNG images from the Tango Project