I was a little hoping you were going to hand deliver the film to a CVS and have it printed AND THEN scan it :)
As for imagemagick, you could make this harder by:
a) Parsing the PNG structure of every image (4-byte length, 4-byte type, data, and 4-byte CRC).
b) Decompress and unfilter the pixel data
c) Convert data to indexed color and quantize down to <= 256 colors if needed
d) Write the GIF container
e) LZW-encode the pixel indices
man what a cop out, I was hoping they would use a text editor to build the gif. that would be way harder.