--- a/src/pyams_portal/zmi/portlets/templates/image-preview.pt	Sat Jan 27 00:42:55 2018 +0100
+++ b/src/pyams_portal/zmi/portlets/templates/image-preview.pt	Wed Feb 07 18:03:19 2018 +0100
@@ -9,7 +9,7 @@
 						   slot_width slot_config.get_width();
 						   base_width 100 / 12;
 						   thumbnails extension:thumbnails(image);
-						   target python:thumbnails.get_thumbnail('800x600', 'jpeg');"
+						   target python:thumbnails.get_thumbnail('800x600');"
 			   tal:attributes="href extension:absolute_url(target)">
 				<picture>
 					<tal:var define="width slot_width['lg']">