In case te don't have Photoshop, te can download it link

Now that te have Photoshop, we can get started.
First te need to click File--->New---> width 800, height 100 pixels. Then te click Ok and te have a background.

Now te click File--->Open and te select a foto te want to open. Then te crop and resize it. I usually choose 110 o 120 height. Now right click on little window and sposta to new window. Now your image is in new window and te need to drag it to your banner. Then te use a little Arrow from the panel on the left and sposta your image where te want it.

You do...

PNG is a strategically cropped layer. te can use it for your icons, banners, random fan art... te can make your own PNG o download one.
link

You drag PNG files same way te do with link

Here are some PNGs, all from Deviantart:
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I didn't test all of these so sorry if some are not very good.