Here is a node group which lets you add a logo over your render. It's driven by render settings (X, Y, scale) so it will adjust automagically. You can chose in which corner you want the logo, scale it, and add some padding to it. Play with it a few minutes, and include this node group in your startup blend file ! (perhaps you will have to recreate the drivers. Right click on Render Width field in the node group > Single from target > left click on the X size in the render settings panel... Repeat for Height and Scale and you're done)
And you can chain this node group to put more than one logo if you want !