OK, I am able to get in now.
I took a look at your photos and it looks like SmugMug is not assigning a “download link” to them. There are some related old threads on their forum that talk of issues with the download links, but your issue is slightly different. Essentially Photonic looks for something called the “ArchivedUri” on a photo. If found, that gets assigned as the download link, and the SmugMug server subsequently returns the photo as a downloadable object as opposed to something to be displayed directly on the browser (in technical terms, the server has to return a specific Content-Type
to directly trigger a download).
In your case since the ArchivedUri is not being detected, Photonic is picking up the size that you are using for your lightbox and trying to assign that to the download button. Unfortunately this image is not returned with the right content type.
I would suggest opening a ticket with SmugMug stating that you are not getting an ArchivedUri to associate to an image for a download.
One possibility is that if your albums are protected SmugMug might be preventing downloadable images from showing up, or you may have prevented downloads in your album configuration.