@akhuebel I could not find a way in Canvas to get to the image options in New Quizzes like you can in the other areas of Canvas. Normally, you can click on the image and "Image Options" will appear which opens up the pane to edit the alt text. I know the Rich Content Editor is being worked on right now in New Quizzes, so maybe this will get added at part of that.
If you are just checking for accessibility, you might be interested in turning on the developer tools if you use Chrome. Once the tools open, click the little arrow in the top left corner of the tools

This will pull up information about whatever your curser is hovering over. This is a great little tool that will tell you the accessibility of elements. You can hover over the image in a stimulus and the "Name" field (you will see a little window appear at the curser) is what is in the alt text. Will not help you edit it but will at least tell you was entered. It is also great at checking contrast ratios for accessibility.
Hope this helps!
Nick