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It can be any size you like, or click  here to see standard banner sizes. Make it something that works with the rest of your website. Click on the Paste tab, and from the menu, select Paste from. Find a picture you like, and click the Open button. Click on the Resize tab, then choose Pixels. Set the vertical height to match the height of your banner.  Move the picture into place. Add as many pictures as you like (and that will fit!) Using the Text tool (the A button), add your name or any other text you like. Use the Select tool and draw a box around your banner. Make sure it's the size you want your finished banner to be. Then click Crop.

Summary:
Create a new document. Draw a banner-sized selection. If you want a colored background, use the paint bucket tool to fill the banner with any color you like. Add photos, pictures and text. Resize your picture if needed. Add your name. Crop your banner. Once you're done, save it!