UPDATE: I'm still not 100% there, but I discoverd an important thing about this question which is that what I'm looking for is called a 'lightbox' so for any wandering people looking for an answer to the same question, that's what it's called
There's a tutorial here, but apparently it's only for v7.0 squarespace and it does not apply to the newest version which is v7.1
Lightbox, of course--not to be confused with this scene from space odyssey, but perhaps I am just as perplexed