If you want to change how all of the images in all of the posts look like -- you can just override the css rules with your custom CSS. This should be fairly straightforward.
If you want to change just some of the images, you can either create a new CSS class and apply it to these images via a new shortcode or with a Markdown Render Hook
I'm creating a blog post and including snips from an iPhone, can I use any inline formatting to size the images and add a border?