Clean up common/ images - moving, deleting, renaming as necessary

@talbs and/or @frrrances - small, descriptive commits specifically describe changes

There are two images that would be awesome to remove:

common/static/images/mit_dome.jpg - used in latex problem template & test fixture
common/static/images/simple_graph.jpg - used in one problem template example.

Seems like it would be better to have just one, generic "image" to use as a placeholder in cases like this, where we want to have an image but what the image is doesn't really matter. Any ideas as to what such a placeholder image could be (or if we have a stock one already)?