Rollover Image in Category Menu

Hi, I have set the category menu items so that there's a rollover image, but when you hover over the menu it doesn't change to the image, only the dotted circle changes colour. What am I doing wrong please?

(By the way your support site doesn't notify me when there's a new comment added to my ticket, so I didn't know you had replied to my last ticket. This might be happening to a lot of people.)

Thanks :)