I use a lot of photos in Squarespace - I am a photographer so thats why!
Anyway, the conventional wisdom regarding Search Engine Optimisation in general has always been 1) name photo files using relevant words (obviously), use lower case only, use hyphens and underscores between words, don't use unusual characters, don't use lengthy file names 2) use photo Alt Text to describe the photo as if to a human, upper case and spaces are fine and in fact better as more human 3) use photo Captions in a human-like manner similar to Alt Text. So far so good.
However in practice that can be an enormous amount of work which I suspect may no longer be relevant.
I create new blank sections (with my own default Format Background and Colors) into which I add multiple photos one at a time. My photos are a mixture of landscape upright and square aspect ratios and I have not found a way to do that mix of ratios using any of the Squarespace supplied Images Sections.
For each photo for adding to my Section Squarespace specifies each filename but notes it as "FILENAME (OPTIONAL)". "IMAGE ALT TEXT" is also available to enter, obviously. And "CAPTION" selected as "Caption Below".
It seems to me that so long as you use Captions with keyword relevant content there is no longer any point in having keyword relevant file names or alt text because Google is using the Captions to describe the photos anyway. I've found this appears to be the case now when browsing some recent expert SEO content elsewhere.
You may think "why not add all three anyway, it cannot hurt". However if the amount of photos is anything from a dozen to a couple of hundred then adding all three is a huge job, particulary as you should not simply copy and paste filenames to alt texts and captions.
Any thought. Thanks.