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

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

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

You do...

PNG is a strategically cropped layer. toi can use it for your icons, banners, aléatoire fan art... toi can make your own PNG ou download one.
link

You drag PNG files same way toi do with link

Here are some PNGs, all from Deviantart:
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link

I didn't test all of these so sorry if some are not very good.