I played a little with the program (from GitHub) and the warning is printed when one of the parameters is 0 (zero).
Considering files .../picwoodpecker/2016_..._0.jpg 568 0 GD Warning: one parameter to a memory allocation multiplication is neg +ative or zero, failing operation gracefully
parameter number is '$small_w' and the second is '$small_h', from the code line:
my $resized = GD::Image->new($small_w,$small_h);
Update: made a clarification.
Best regards, Stefan