There is another possibility. Use a DVD player which has an USB interface and a composite video output to connect to the ATV transmitter. On a USB stick you can store video test patterns (or other pictures) and show them in a slide show for example.
A small modification to the Telefunken DVD player makes a composite video signal available (the red wire)
Contact 17 and 19 of the SCART interface provides the composite video signal for you:
On the right hand side you see the yellow cinch plug that transports the composite video signal. I installed the cinch connector on the front of the DVD-player for easy access.