Copyright permits the owner of the copyright to determine how his work is used. One does not have to sue to have copyright enforced (I have, for example, had web host remove an image that was a result of an unscrupulous plagiarist ripping off an image of mine, doctoring it with image processing software, then trying to pass it off as his own "artistic" work).

Many people will put statements on their websites that indicate how the contents may be used. It is incumbent upon us to abide by those statements and, where none exists, to contact the copyright owner before using the material. This includes the posting of images using bbcode tags or posting direct links to images. It has often been the case that "popular" images that have been linked in this manner have resulted in the owner exceding his bandwidth limits and thus causing him considerable inconvenience.

OTOH many people want their images to have as wide a distribution as possible (good for the ego!) and are more than happy to have it widely used provided proper credit is given and the original URL of the image is published with it.

If in any doubt at all, ask first.

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Stephen