I would certainly say yes. They're using your bandwidth to serve another one of their customers, for free, without compensating you in any way. Whereas loading

I would certainly say yes. They're using your bandwidth to serve another one of their customers, for free, without compensating you in any way. Whereas loading a 150MB image from their server is using their bandwidth (that they're paying for) to serve you. Legally, I have no idea. Morally, it'd be nice if they at least asked and actually had a working site if you say "no".