hwa630

You can find guides like this that can be helpful:
https://insidethesquare.co/css
Different image types have different classes, and every individual block has it's own ID which you can find either by inspecting the code or using a Chrome extension. 
https://squarefortytwo.com/squarespace-guides/find-an-id-in-squarespace-71

Nope, that was me missing something:
.sqs-block-image img { opacity:0.6; transition:ease-in-out 300ms; &:hover { opacity:0.9; } } I deleted a semicolon, just to demonstrate how easy it is to have mistakes in code.....or maybe just by accident!

This will make images have an opacity of 60% and 90% when hovered on.
Place in Custom CSS under the Design dashboard option.
.sqs-block-image img { opacity:0.6 transition:ease-in-out 300ms; &:hover { opacity:0.9; } } You can replace .sqs-block-image img with anything you want, block IDs list sections, gallery images etc.

You can add a hover effect to any block in CSS, and set an opacity. You would just need the block ID if you wanted to target one individual block, or a class if you want to target all image blocks on a page or more.