I played a little with the program (from GitHub) and the warning is printed when one of the parameters is 0 (zero).

Considering files[0] .../picwoodpecker/2016_..._0.jpg
568
0
GD Warning: one parameter to a memory allocation multiplication is neg
+ative or zero, failing operation gracefully
[download]

The first parameter number is '$small_w' and the second is '$small_h', from the code line:

my $resized = GD::Image->new($small_w,$small_h);
[download]

Update: made a clarification.

Best regards, Stefan