Source: Mxing it
Notice the right of the image has filled with empty or transparent pixels. If you are using a variable-width theme or template and using the image for the background image of the header area, you need to make it at least 1800 pixels, even though most browsers will only see the first 1000 or 1200 pixels of width.
8. Now let’s insert a background color / layer. Go to Layers > Add New Layer. The Image will remain unchanged but note the Layers window now shows a new layer, “Layer 2”.
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