Two reasons:

• You don't have any captures in your regexp, so $1 is never given a value.

• The do is executed before the match. The match might not be executed at all, depending on the value returned by do.

Fix:

while ($get_gal =~ m#(http://{...}.jpg)#g) {
   my $image = get($1);
   my ($height, $width) = imgsize(\$image);
   push @found_images, $1
      if $height <= $max_height and $width <= $max_width;
}
[download]

Bunch of alternative styles: