A media source was set up named ResourceMediaPath, combined with MIGX. I think this was done along the lines of the online manual at
http://rtfm.modx.com/extras/revo/migx/migx.tutorials/migx.use-resource-specific-media-source-and-multifile-uploader
Chunk named 'header-afbeelding':
[[+image:stripString=`/en/`:phpthumbsup=`w=1280&h=370&zc=1`]]
Code to display image:
[[!getImageList? &tvname=`header-afbeelding` &tpl=`header-afbeelding`]]
I created a new TV to hold an image, without MIGX (input set as 'image'), unfortunately this fails as well. No images are shown in the manager or front-end. Uploading images seems to work fine.
PS: The system was put together by someone else, I'm still trying to figure things out. Weird thing is that it all worked fine on my dev server. And even on my clients previous website running on the same server using the exact same setup. I checked permissions etc, but that seems to be ok.