SOLUTION: Go to the Files page and select files by clicking to the left of the file name. You can use command-a or ctrl-a to select all visible files. Next click the icon at the top of the page that looks like a cloud with a lock on it. Change the settings to "Only available with link" and "Institution Members" (no "Public" in you are using LockDown Browser). If you selected a lot of files, you may have to click Update several times. Unfortunately you can not select a folder and have the settings apply to the files in the folder, but have to select each file. This works for all existing files. If you add files to folders, or upload them while creating a quiz, you will have to go to the Files page and change the settings for the newly added files. If you upload a file while creating the quiz, the files go into the Uploaded Media folder on the Files page. I find it best to give the images or graphics descriptive names before putting them in Canvas, so I can find them again, using the search box at the top of the Files page, or the search box on the Insert > Image > Course image (or Upload image) screen.
