The center of the reel is a great place to put an extra picture, words of wisdom, company logo, etc. BUT please remember, you cannot see the center image inside the viewer.
The viewer is designed to show you the frames of your reel.
The eyepieces that you look through are only big enough to show the individual frames.
Use this image as an example...
The reel has been made transparent to show you how it fits inside the viewer....the eyepieces are outlined in black.
See? Makes sense right?!
The only way to see that center picture is if you pull the reel out of the viewer.
So, all in all, have fun with your center art. Just don't put anything pertinent to your message there. You never know if someone will actually take the reel out of the viewer or not ;-)