Unfortunately, there's no option to do this in Squarespace, in ANY of the templates. It's a serious limitation of Squarespace.
Back in Squarespace version 5 (about 8 years ago), this was possible. But it hasn't been since, for some reason.
At least, not without some Custom CSS code. But even then, I haven't found a CSS solution that works reliably, or with all devices. It'd be great if someone had such a solution now. It's a deal-breaker for art and photography galleries which benefit from captions. Text should not obscure or compete with part of an image.
All that said, I suppose you could try one very sloppy fix that has its own issues: re-create all of your images adding a bar of white space at the bottom of each, and then when you set captions to appear toward the bottom of an image, it'd look as if they were really below the image. But if anyone looks at the images separately, they'd have those weird bars.