WMS and setting image format

I have a installation of geocortex ess 3.5 and Silverlightviewer 1.3.

- 1 cached Mapservice of Arcgis server

- 1 dynamic mapservice

- 1 extra WMS.

This WMS is standard requested in PNG24 (according to firebug). Within the RESTmanager I cant change it to JPEG. Directly editing in the xml file of the imageformat parameter is possible, but will not result in requesting jpeg images. Also changing the WMS-url with adding the format parameter will not work.

Problem is: My images from WMS are between 3 and 12MB per image in PNG format, so I definitely need JPEG (size up to 1 MB). The WMS will send JPEG images when directly requesting an image through url.

Is it possible to force the restmanager/silverlightviewer to request JPEG?