Is it still blank after selecting the image in the list at the top, or do you just mean it's blank when you re-open Preferences?
The preview only shows the selected image. (i.e. Just the one image, if any, that you last clicked on in the list. It doesn't show a preview of what Opus will look like with all configured images or anything like that.)
If that was the misunderstanding, have a look at the FAQ: Understand and Configure Background Images.
BTW, you should never have to restart Opus to change image settings so don't worry about that.