I have similar problem except it happens equally in lessons and in labels in the course. If I use broadband internet connection then I see all images OK but if I use mobile internet connection then I do not see these images. So the clue is - the kind of internet connection you and your colleague uses.
While I upload images I always give them the name so I know where they are on the page. While in Firefox, when I use SHIFT A I prompt to display all images and they appear.
If I run mouse over the name of an image the field underneath behaves odd and it blinks but if I use SHIFT R I prompt for a single picture and it appears. I tried to change internet settings responsible for image display but it did not help in any way.
Not sure SHIFT advice helps you in any way.