Thanks for replying so quickly, awilliams! The other teachers were having locks show up on their images, so I forwarded your response to them and it should work.
I looked in student view and the pictures are showing up, then logged in as one of my students and the pictures are showing up for her now as well. I also gave the quiz in the morning and none of the students said anything, so maybe it was a strange temporary glitch? Here's the icon that was showing up instead of the pictures: 
I uploaded the pictures while making the quiz by going to "Images" on the content bar on the right side of the screen, then clicking the "Upload a new image" button and uploading the images (they were png's in case that might be part of the problem), then clicking on them to put them in the rich content editor.
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