It works for me, or rather, the following does:

use strict;
use warnings;

use Data::Dumper qw( Dumper );
use Image::Size  qw( imgsize );
use LWP::Simple  qw( get );

my $this_url = "http://images.imagefap.com/images/full/5/190/190037365
+.jpg";
my $image = get($this_url);
my ($height, $width) = imgsize(\$image);
print "IMG $this_url is $height and $width\n";

__END__
IMG http://images.imagefap.com/images/full/5/190/190037365.jpg is 507 
+and 760
[download]

Make sure $image is defined. Maybe an error occured during the download?

print(defined($image) ? "Ok." : "Error!", "<br>");
[download]

Oh! And make sure your version of Image::Size accepts images passed as \$image.

PS -
foreach my $img (@image_urls) { $img =~ s/thumb/full/ig; }
can be written as
s/thumb/full/i foreach @image_urls;