You need to have the logo saved as a png with a transparent background.
Then go to sony vegas(I assume you're using this) and drag your video down to the second or third row. Then place your pic in the row above it. Drag it out to match the length of your video so it will be there the entire length of the vid. Then there will be a bar at the top of the pic that controls the opacity. Take that down to whatever you want.
Thanks for the input inspire will look into that. I'm not sure XXLRay, is that the one where it's at the bottom of the video and if you hover your mouse pointer over it, it comes up with a subscribe button? (e.g. In this video: Skyrim Remastered - Why Skyrim? This Is Why! - Camelworks) sorry I cant post the actual link.
I would use a watermark so if somebody uses your video in their video your watermark is in the bottom.