If you own copyright in a piece of intellectual property then you own copyright in any version of that property placed on the Internet. The Internet does not remove or reduce any of your rights. What does change is the practical position. Anything you put onto the Internet can be copied easily and without loss of quality. This is a difficult area for museums.
Plagiarism is a worse problem for images of works of art than for text.
It is relatively easy to tell when one text is a copy of another, it may be much harder to prove that one digital image of an object is a copy of your original photograph. A special concern for art museums is the number of sites now offering images of works of art - some of these are plagiarised copies, others legitimate from third party photographs. It will be difficult to tell the difference. In some cases the images are of poor quality, which is one problem, in other cases they are of high quality, which is another. Scanning a slide takes a few minutes at most.