I'm trying to change some of the images using the weblink designer, and after I change them and open the page this prompt displays (also appears in Weblink Designer when saving the changes):

No credentials I enter into this login are considered valid.  When I click cancel it continues to load Weblink but the updated images are all broken.

I put all these custom images in the Laserfiche\Weblink\Weblink\Web Files\images folder, so I would expect the service running Weblink would have access to the files.  I've even explicitly granted full control to the everyone group.

Any ideas?