I have put images in the assets folder:
However the (shift command m) media command is not coming up with the images I have put in the folder:
All the images are PNG and were just dragged into the the assets folder.
You need to check/enable Target membership for each image that you want to use in your project. You can do it by selecting the image and checking first option which will be same as your project name.
Once you enable target membership clean and rebuild the project.