Images absent

Hi. I hope that you can help me with a recent difficulty on my Moodle site.

When I go into editing mode, certain images do not appear. (See below)

I copied one the the image's url :

http://edugestion.usenghor-francophonie.org/theme/image.php/afterburner/core/1403188933/t/assignroles

This image does not appear. But it is present in my moodle site.

Please advise me on this problem.

Richard