Image module thumbnail lightbox links to thumbnail

I have a view with teasers of image thumbs produced by the image module.

When I click on one of the thumbs, I get a lightbox, not of the original image as I had set it, but of the thumb (and using an incorrect url too so it just shows an x).

If I do a rebuild derivative images on the image itself, it suddenly works again. But it can't be a derivative image issue as I had set the original image to be the one to be in the lightbox, not any derivatives.

Please help. Thanks.