The pictures are relatively large (say 2500x2000) and fully uploaded. In the post, Discourse has resized them to 666x500px with width and height options added to the img element.
If I remove the tiles div, the images appear in the post but are not selectable (can’t click to enlarge the image). I would presume they are not being placed in the lightbox viewer.
The option create thumbnails in the Discourse Settings is enabled.
Version is 2.0.0.beta7 +37
Other elements in the post include several paragraphs of text, an image not in the tile element at the top of the post, a onebox’d video URL to YouTube, and footnotes.
Tested it on try.discourse.org and the image was placed in a lightbox without a problem. I tried running the site on safe-mode to see if the images would load in a lightbox, but no change.