Admins can set the default thumb size already for the "thumb" attribute on the IMAGE/FILE BB-Code. If this size is not sufficient, the wiki is designed to automatically generate other thumb sizes based on an admin setting (which doesn't exist yet). Other requested sizes are supposed to go into a special "moderation" queue on a per-use basis (this queue also doesn't exist yet).
In order to achieve a thumbnail of a "main" image, however, there first needs to be a way to define a "main" image without having to actually parse the page.