In Create-A-Course you have the ability to add individual slides after the course is already created. To do so, open your course and click on one of the plus buttons (+) when selecting slides.
A new window will appear. You have the option to add a new slide Image or Video, which can be selected with the Slide Type dropdown.
If adding an image it will need to be at least 800px wide. For videos you have the ability to add a caption and description file for accessibility purposes.