This is how I printed on Flash Paper.
Printed out the picture on an A4 sheet, I got it so the picture was central to the paper.
I then used Prit stick lightly round the edges of the flash paper and placed it over the picture on the A4 Sheet. Put it back in the printer and hey it worked.
Another way I thought of but not tested.
Rather than use prit stick print the picture on the A4, then make a window with more A4 paper and slot the flash paper in. make sure the Open end go's through the printer last. Ok quick Blue Peter demo below.
http://www.101tricks.co.uk/demo.htm
I hope this helps..
