Just click the upload button above. You can upload as many images as you want all at once.

By default, the primary map image is the one at the top of the files list. You can override this by renaming any of the images with "0" in front to bring it to the top.

For videos, make a new tab in the media section and insert the video code using the video ID:

You can insert videos similarly with vimeo, googlevideo, and gametrailers. Take a look at the inserting videos guide for more examples.