I don't think the Media module (which TinyMCE uses via the MediaPicker module) exposes an event when media is uploaded, so that's going to be a problem (unles you modify that code to expose an event via the event bus mechanism: then you could write a handler
that generates the thumbnail for you).
Or, you could do it the other way around: instead of generating a thumbnail during upload, you generate a thumbnail during the initial request of the image. For that you could use the imageresizing.net and its disk cache module (it's not an Orchard module,
but a .net library containing http modules and handlers).
Once installed and configured correctly, all you would need to do is append some querystring parameters to your image url. For exampe, suppose you wanted to display a 100x75 thumbnail of an image of some arbitrary size, the "src" attibute could have this
What will happen is that imageresizing.net will generate a thumbnail (and if you enabled the disk cache, store it on disk for the next requests) and serve that one instead of the original image.