For a list of all components and their purpose, please refer to this list of components.
When published, a media block looks like this (well, it will look like whatever picture you've chosen; we've chosen an arrow here):
When you are adding a new Media block, you'll see this screen:
To add media, click Add Media (we'll use a photo as an example) and then choose what to do next:
If you wish to upload your own photo, click Choose File, browse to the file location, highlight it, and click upload. The photo is then added to the library that you see here.
If you know the photo is in the library but you don't see it, use the filtering and sorting options to look for the photo.
Regardless of how you get there, when you see the photo you want, click the little box to select it.
Then click on the Insert Selected button.
The photo will be inserted into the Text and media block, as you see below (we've selected a picture of a yellow arrow from the library).
To change that picture, click on the 'x' in the upper right of the picture to remove it and then run through the steps above starting with "Click Add Media".
At any point, if you decide you wish to remove the Media block, you can click on the ellipses in the upper right of the block and then choose Remove.
There is no individual 'Save' button for a component. Components are saved when you save the page.