Problem:
Users should be able to upload high resolution video clips from within an app (fat client) that runs on their system. inside this app, i need an framework or lib that is capable of converting an video into an low resolution version.
and on the other side, i need something that is capable of building up video right away from images, animating them in a way that they look like videos.
You don't say which system, so I start this with an answer for Linux: Have a look at phonon and gstreamer. If you don't mind a command line interface, check out avidemux and mencoder (part of MPlayer).
Note that phonon is also available on Windows and Mac.
I would recommend the following:
If you need a cross platform solution I would recommend the commercial MainConcept CodecSDK, which has some of the best video codec implementations around.