You could do this in paint as well. You do not need something overly expensive such as Photoshop.
For example, on Windows 7 simply clicking the image will show you in properties that the original bg.png is 1650x800 pixels (right click the image and go to properties to see this if you do not see it because of your window preferences). You could then right click your wanted picture, open with --> Paint.
In paint, in the Image area, click Resize and type in 1650x800 pixels to get it exact, then you just simply click save as and use the pull down menu to select PNG, then type bg in the name field. This way you're not losing your original image and you have one just for the background. At this point you could rename the original bg.png to something else so it would not be used.
