Would like to use a photo’s filename as breadcrumb rather than title. Is this possible?
The only way I know of is to leave the metadata Title field blank. The image breadcrumb will then default to the filename.
There may be a phplugins solution but I don’t see a hook for breadcrumbs. Maybe @Matthew or @Daniel will have some ideas.
Thanks for the quick response, Rod. Was hoping for a solution other than not using the Title field. We make fairly extensive use of this field for photos we publish to Flickr, most of which are also published via Backlight. 
There are other ways to display the filename. You can use the caption area or the metadata panel in the album template. You could also use that phplugins function I referenced to display the filename above the image (and below the breadcrumbs).
And, as I mentioned above, Matt or Daniel may have other solutions. I started thinking of a way to use jQuery to do it, but quickly got my brain tangled.
You can use phplugins to create the breadcrumbs.
A while ago, I wrote about adding breadcrumbs to non-gallery pages: Adding breadcrumbs to non-gallery pages (updated August 2020) – lab.danielleu.com. This could serve as a starting place. You can use $_SERVER["REQUEST_URI"] to extract the gallery path and then create the breadcrumbs trail from there.