If you're adding a logo for the sake of preventing image theft, it might not be such a great idea. Watermarking your images won't necessarily prevent people from stealing them. Adding a logo to your images serves more to increase the exposure of your brand, when people view your images or share them across the web. Companies creating infographics would be a good example of that.
Here's an article that would be relevant to this topic: http://blog.photoshelter.com/2010/09/watermarks-protecting-your-images-or-damaging-your/
Hope that helps!