Just a quick update: Dragging reference images into the Blender viewport also works with Blender 2.8 where the images are loaded as attachment to an empty so that image properties like size, position and opacity are set in the empty's "Object Data" tab in the "Properties Editor". Nevertheless, for this course I will use Blender 2.67 in order to focus on the creative part without need to search for equivalent settings in Blender 2.8.
Quoting myself (April 14th 2020):
I will use Blender 2.67 in order to focus on the creative part without need to search for equivalent settings in Blender 2.8.
After some months of gaining sufficient experience with Blender 2.8 and 2.9 I'm confident to quickly find the appropriate settings in the Blender 2.9x GUI. It can only happen that some things don't work in Blender 2.91 as in Blender 2.67. An example is the "Layer Brush" which has a "Known Issue" (according to the Blender developers here) concerning the "Set Persistant Base" functionality as I've described it here. See also here Kent Trammell's article "Learning 3D through various Blender versions" with the recommendation
Always use the same version of Blender that you see in the video.
if you want to be sure that everything works as demonstrateted in the tutorials and courses.