I would absolutely agree with David. No hotlinking, for sure, because even if it is unintentional it is stealing. You don't know how much bandwidth this website has, and if you push it over the limit if say, your website is very popular, then their site goes offline. Can you tell I've had this done to me?
Also, yes, you ask for permission to use an image. Imagine if somebody took one of your photos to use as a reference for something, but without asking. Would it bother you? I look at it as having control over an image. Unless the image is under creative commons licensing, it's completely copyrighted.
Have you tried specifically looking for images of what you need that are under creative commons? I believe flickr has a search function for this specifically. You can also go here:
http://creativecommons.org/
I believe they have a search function.