Put a check if wrapper is blank.

[archergod]

In case the wrapper has no image tag, it was generating error and halting the rest of execution, with this patch it will by pass such blank wrapper.

[GianlucaGuarini]

I think this should be enough:

if (!$img.length) return;

[archergod]

well $img was coming as undefined so it will again generate the undefined error with that statement.

[GianlucaGuarini]

$img can never be undefined it's a jquery selector instance that is always an array. Sure your problem is not somewhere else? Could you make a jsfiddle please?

[unbreak]

Why dont use:

$('.bwWrapper img').parent().BlackAndWhite...

?