It really depends what your site is about. If you have a highly specialised site then maybe not, but if you want to stand out in a crowd then definitely
You cant say important but better to have them. Even when i provide web 2.0 submission to my clients I ask them to include images and video. It solves the problem of spamming to a great effect
Its always good to have contents on web pages but if not then at least you should use proper alt text for the images.
Content-based" means that the search will analyze the actual contents of the image rather than the metadata such as keywords, tags, and/or descriptions associated with the image.