Generating multiple files from a single image.

Edit Subject

Hello, I'm a total newbie to EE and just got Assets and some other plugins and not sure how or the best way to accomplish the following scenario:

The user uploads an image (800x600), needs a thumbnail and a medium size image. Thumbnail 100x75, Medium 350x263.

And pushing my luck:
Same will apply when uploading multiple files, each file needs to be assigned to a thumbnail and a medium size image.

Does this makes sense?

Any help would be greatly appreciated.

Thanks