There are three types of backgrounds you can choose from:

• Transparent Background - All background pixels remain transparent.
• Solid Color Background - All background pixels are filled with a single color. Use the color picker control to change the fill color.
• Image Background - An image will be displayed behind the original image. If you've copied an image into the clipboard, you can paste it into the background layer with the Image from Clipboard button. You can also use the Image from File button to select an image file.

You can also change the size of the background by checking Change Background Size. When this is checked, you can use the width and height controls to adjust the background size, or the resize widget over the image to adjust the size and position of the background. Once you're done using the resize widget, click Apply Size.
The Alignment buttons are used to align the original image to the background. So for example, if you want your original image to be centered in the background, click the horizontal center alignment and vertical center alignment buttons.