You can have Shades display another picture or color as a backgroud. This is another "layer", completely independent on the picture. It shows the background in place of the transparent color (E7FFFF or 231,255,255), so you can "see through holes" etc. This can be useful in exactly three cases, if you want to...
The background dialog consists of two parts, one for color and one for pictures. These are separated by a horizontal line.
The color part includes a text field into which you can type the background color in hexadecimal notation (preceded by FF for trasparency - unused in this dialog), a button which brings up the standard Windows "select color" dialog, a small rectangle with preview of the color and finally a button to restore the normal background color.
Main control of picture part is list of loaded backgrounds, below which is always shown preview of selected item. The treee buttons to the right are self-explanatory: add new background (from PNG picture), delete and rename the currently selected one.