When preparing my images for the web i normally work to the standard of 900px high x 1000 px wide, 72 dpi and not exceeding 400kb (so that files will load quickly).
I've just been going through the Squarespace tutorials and I understand the upload as 1500 px wide and don't worry about the resizing for different screens thing (as squarespace does that) which sounds great to me!
However does that mean I still have to aim for circa 400kb file size per image or does it not matter as gallery/image blocks will be resized by Squarespace? (and therefore export @ 100% quality from photoshop and not compress it down)
I'm using the wexley template in case that is important!
Cheers!