So I asked them:

Quote

An image of mine is used as a background at this page:
http://pianoprofessionals.com/copyright.html
at 1300x986pix
Is such a large format allowed in web pages?

And they replied:

Quote

Thank you for your e-mail. Yes, once they purchase the image, they can enlarge it as much as they want/need.

Did they reply to my question?  Did I ask the wrong question?

This is correct. In this particular case, they are not allowed to display the image more than 640x480 pixels. We have sent this email to our special department in order to review this. We appreciate your patience and support.