The answer to this ought to be straightforward, the publisher is the party who commits the infringement.

However publishers often do not accept responsibility, and will blame the problem on a web or graphic designer whose services they have used. This may even be true, the publisher may have no idea that his site is displaying your work without permission.

Do not be diverted into chasing the designer or whoever the publisher claims is responsible. Legally your claim is with the publisher. They may in turn have a claim against their designer, but that is not your concern or problem. You will only end up being the victim of both whilst they squabble about whose fault it is.