Hi,
Ok, I got the problem here.
When I add the images to the description, I get the plugin to work.
When I add the images using PHP (i.e. dynamically from the DB), it doesn't work.
The reason is that Mthumb is a content plugin that works "onPrepareContent" and when I add the images using PHP it's "onAfterRender", so the plugin does not apply on the images I add...