Side by side image thumbnails in 2016 theme

Hi All,
I can’t find a theme specific forum, hope it’s OK to post here.

[ Moderator note: moved to Fixing WordPress. ]

I’m using the 2016 theme.

I want to have two thumbnail images, side by side, (these are a before and after pair). There’s space but I can’t seem to find a way to stop them appearing one above the other instead of side by side! Seems basic to me…

Is this possible please?
How?

Nick