There is a problem with images uploaded by students into wikis via the file uploader.
These images will disappear after a while.
A "broken image" icon will be shown and the original image file is no longer available on the server.
This issue will only occur if
a) the uploader is used AND
b) the image is uploaded by students.
External image links added by students OR image files uploaded by trainers or admins are not affected.
This issue seems related to
MDL-58157 , however this bug report was closed without a solution.
MDL-58157 Images that disappear from the wiki