In the thread App: External map styles - #16 by rumbrummer I inserted two small pictures - they can not be zoomed just by clicking / tapping on them.
In the next answer I added also pictures in the same way (snipping them to the clipboard and pasting them by typing Ctrl-V), afterwards set zoom to 50% - this pictures can be zoomed by clicking / tapping on them.
What is the criterium for zoomable yes / no? Is it just the size ?
Small images are displayed with their original size.
Large images are displayed with a smaller size to fit on the page.
(click to open them with larger / original size)
Hmm - but I can also choose the zoom of small images during insert - I will insert same image with different zoom:
100%: 
50%:
Both are the same image - but the 2nd one I set to zoom =50%. Nevertheless I can not zoom it when reading the post by tapping / clicking…
My first gut feeling ist that this is an arbitrary cut-off point somewhere in the Discourse (the forum software that we use here) system, where they determine whether a picture is large enough to warrant a click-to-zoom
However, I also found a post from somebody with similar curiosity to you
And they concluded that is has something to do with how you upload your picture? Dragging it in vs. uploading via the button might make a difference?
I have not tested this yet, but the post is super old, so I think Discourse would have fixed this already. So I strongly believe that there is simply a size cut-off somewhere