Bandwidth can be a real problem for some people. It's always a good idea to ask permission for any image if explicit copyright/permission info is not posted. Many people won't mind if you ask for permission to copy to your server with credit and a link back to them. That solves both the bandwidth and permission issues.
Never hotlink an image without permission. This past spring I found someone had hotlinked a high-res picture on my site and it was sucking up huge amounts of bandwidth. Most people don't realize that hotlinking costs someone else money, bandwidth, or both.
(Because someone always asks when I use that term, "hotlinking" means embedding an image from one site in another web site. Every time someone brings up the page with the embedded image, it has to load in from the other server. You can imagine the load on the other server if a lot of people hotlink the same image. Of course, sites like Flickr or Photobucket are designed for this, but you still need permission to host an image that isn't yours.)