I think the issue is more about which info lightbox uses to caption an image (the caption bar which appears on mouseover/touchscreen-press, and always visible in the gallery viewer).
The lightbox caption bar currently appears to use info from a title attribute in a.class=lightbox, where that title attribute holds the image’s filename, plus image resolution & filesize (the image filename appears again in the mouseover tooltip, where it seems lightbox is generating this tooltip using CSS, as the img tag has no title attribute).
Preferred would be for lightbox to use the img alt data from markdown in the caption bar, i.e. from [alt text]($img_url), with perhaps the complete filename/resolution/filesize in the mouseover tooltip (info which is potentiallly useful for a desktop user, not so much for a touch-device user).