watermark incoming videos

On my site I save the original video file and a converted flash version. I want to set the size of the converted flash, and to watermark the flash with a semi-transparent watermark. The mark should include my site name, and the uploader's username. In order to make the watermark uniformed on every video it may be necessary to re-size the flv unless you can use variable percentages to locate and size the watermark. I already have a working upload process, just need the watermrking included in the [login to view URL] file.