Gimp is the king of free photo editing software. It's similar to Photoshop in capabilities. So it's more than capable of putting on a watermark, but it's complex too.
If you just do a search for "watermark photo free" you'll get dozens of hits. I saw one for a batch editor if you're looking to add your signature to all your photos. I also saw a link on the first page of the search that said "Top 10 Free Watermarking tools". So you can get someone elses opinion on the options. These free tools are going to be basic editors, some might have adds or try to get you to upgrade, but if all you're looking to do is put a signature on some photos, this is the route I'd go.
Canon also gives you a bunch of software with your camera. I never look at this stuff, but I'd expect there's some sort of photo editor in there? Adding a watermark is a very simple operation, I'd expect just about any photo editor can do it.
And lastly: I'm not trying to steer you away from Lightroom, I think it's hands down the best program for the digital photographer to own. But if you're not going to use it for anything other than the watermark, I'd go the free route.
