The buttons that come with Nero are pretty standard and after a while they appear to be a little boring. With Photoshop or any other program that can save out PNG files you can change all that.

If you open Nero Vision's buttons folder:
C:\Program Files\Nero\Nero 7\Nero Vision\Buttons
You can see the basic idea of how they work. The red areas are transparent and the green areas are where your thumbnails appear. Anything else is treated as an image and shown in the menu.

Red: ff0000
Green: 00ff00

The buttons that come with Nero are all different sizes and not having experimented with them much I'm not sure how big you can go although having an image too big would lose detail in the menu due to the image being reduced to fit the screen. A standard DV PAL screen is only 720x576 and not all of that is visible - the outer edges form the border and are hidden from view (noticed the dashed rectangles in Nero Vision and wondered what they were for?)

I've gone up to buttons measuring 300x250 and everything looked OK.

Thanks for your guide Zeb