I find that the images in galleries are hidden in old, until you click the Play or Plus button (or whatever the subreddit css defines for that) that's under the title on the comments page, just to the right of the up/down vote buttons and the score. Single images seem to show up just fine for me.

I'm chalking it up to my preferences; I have "Show Thumbnails" disabled, and I suspect that's it, but I haven't experimented to verify.

Interesting - I also disable custom themes, maybe that's why I don't see that button?