Adding an image into a video (watermark)

Hey there, I need some guidance regarding the best way to go about my project.
Ok so the aim of the project is to add a watermark to a selected video as fast as possible.
The functionality:

• Select video file (format .mp4) **(More if possible)
• Select an image (format .png)
• Then the selected image is added to the video in the selected corner.
• The main aim is speed - has to be able to complete this task as quick as possible.

I don't know if this would be possible with out having to re-encode the video?..
I have come across this library that looks like it may be of some use.. http://splicer.codeplex.com/
Also these links: watermark-on-video
http://tmhare.mvps.org
http://www.mediasoftpro.com
add-watermark-in-video
I guess the main question is what would be the most efficient solution/tatopic/167/ke on this project?

Main thanks.