Ok reno, I set my Videobox attract to the same settings that you have and I had the same problem, after a little investigation I found the problem...
Click on the 'Show advertising images' button and a dialog box will open, you have to then manually select your 'Advertising Images' folder and click ok, this 'sets' the folder Videobox should look in for images.
Give that a try and let me know if it works for you.