Can't think of any examples I've seen, but images with transparent regions are pretty simple to do in a proper image editor.
Making the transparent area is as simple as using the Eraser tool, or Masking if the editor has this feature. You then have to save the image in PNG format, 32-bit color. (In Photoshop, you use the 24-bit PNG option with transparency enabled, which actually produces a PNG-32.) The image can then be overlaid using 'absolute positioning' in CSS.
If you need a single image and not a stack, just use layers in your image editor, along with the aforementioned eraser or masking.