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.
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My suggestion would be to re-insert the image. When you insert an image, a dialog box opens asking for the alt text as shown on the right side of the screenshot below.
I know this isn't ideal, but it seems to be the only route to go unless you are using a tool like ALLY integrated with your Canvas.
I have gotten so dependent on ALLY, that perhaps I have forgotten some technique, or it has changed. Perhaps somebody else will be more helpful.