...between scenes during editing? Also, is there a site that gives free ones?

It depends on the program, but for most it is just called still image. I.e. in iMovie, you have an option to create still image. They are also called transitions, as they transition from one scene to another.

Those are part of what is called the transition. And you can use any image you wish. Try flickr for a broad range of still images.

Watermarks.