My apologies! I take back what I said yesterday. I evidently did not follow the instructions thoroughly enough.
I had the input and output files both set in the same destination folder, that is to say, where the program is sitting.
This time around I had both the input .png and the output file set one folder above where the program is sitting, just like you have it in the GIF instructions above.
Slightly unexpected input/output behaviour but manageable.
Cheers!