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

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

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

You do...

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

You drag PNG files same way you 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.