Many sites allow copying of their images, but this does not necessarily mean that they allow direct linking to their images too. Direct linking or hot linking is linking to an image or any media files on someone’s web page on their server, i.e they will have to pay for usage on your site. This is a form of theft as web masters have to pay on the basis of amount of data transferred.
Hot linking does use up bandwidth of the website owner, every time an image or media files in laded (though on another site through linking) bandwidth is used up.
Many people who link images aren’t even aware of the fact that they are unknowingly causing a kind of theft. If you want to put up an image on your site, it is important that you use free picture hosting sites like tinypic or photo bucket to host images and not link to the picture directly.
Webmasters can find out if their files are hot linked and can retaliate badly, they can change the original image with a funny nonsense type images which will now be visible on your site, or even report you for going against their terms and conditions. Whats more webmasters can even replace the images with their logos as a form of free advertisement on your site.
So the next time you want a picture from a site on your site make note hot linking is a form of theft!!