I seem to remember I could once do this by right clicking on the image and a box would pop up for me to enter alt text, but this is not working now. I don't see any option that works for inserting alt text into an image in the menu options above the text box in the quiz question creator either. I will insert a screenie to show what I see.

Oddly enough, when I use the check accessibility feature, it tells me accessibility is fine, in spite of the lack of alt text.