Altering specific .jpgs with .css

Hello,

Still pretty new at this, so while I know what I need to do, I’m missing a key piece of info that I can’t find on the forums already. Perhaps I’m just not asking the proper question.

I want to add makeshift borders to the homepage of a website I’m tinkering on (it’s a self education project for me as much as anything else) and I’m intending on doing this by adding the image twice to my homepage in the editor. I’ve figured out how to do that, and I understand I need to alter the attributes until I like them with .css, but I don’t know how to create a .css that only affects one for the right and another with mirrored specs for the left. Like, I know how to create .css in the appearance tab, its just the “I want you to only affect this instance of the picture” that I can’t figure out.

Also, is there a reference of the keywords that can be added to the .css and what they do? I’m learning and can grasp the concepts but would love a cheat sheet to gloss over when I’m looking for a specific result.

Thank you in advance.