To enable the use of background image on your web button all you need to do is click on the "Choose" button and select the background image you want to apply to your web button. It will then automatically be set as background on your web button. Finally, you can also define its position as tile, stretched or centered accordingly.
You can see the relevant changes directly through the preview window. Background - Color
If you do not want to insert an image but instead want to add a color as background, do not click the background check box and just select the color you want from the color picker to apply it accordingly. Automatically the color you selected will be set as a background.
You can see the relevant changes directly through the preview window.