Adding image guidelines to Jenkins contributing guide

This is partly inspired by recent pull requests and my own work in the Jenkins project over the last year.

The Blog post and Logo sections mention images in various capacities, but there are not dedicated image guidelines. With the growing Jenkins community looking to get started & contribute, I wanted to start a thread here for discussion, as I am suggesting some additions to the Jenkins contributing guide for images.

My pull request is located here, and while it is nothing ground breaking, the suggestions should help with consistency and clarity when adding images to Jenkins documentation.

I also wanted to highlight/make a note of alt text, as this is important for accessibility purposes, and more often, the alt text is very limited/basic and does not necessarily provide the context/information needed.

I am incredibly open to feedback and discussion, so please don’t hesitate to share your thoughts! My goal is to ultimately make the contributing guide clear and helpful for any contribution a user could make. This is one of a few pull requests that will be submitted, to add additional guidelines for documentation and other submission types.