To make brushes in Photoshop you start with a clear background and one empty layer above that. In the empty layer any black or gray pixels will become the brush, white pixels will become transparent. This works with any grayscale image. You could take a picture of the Eiffel Tower, desaturate it to make it grayscale and then click on Edit - Define Brush Preset and then have an Eiffel Tower brush. The Brush Editor is where the magic happens, though. You can increase the spacing, make it rotate, make it scatter about, change the size, and all sorts of other stuff. The one thing that you can not do in Photoshop is make color brushes...for that you need an image pipe or image hose like those found in GIMP or Paint Shop Pro. And in those you can use multiple images to make brushes.