If you want to add text or a graphic to your photos, use Lightroom’s watermarking functionality. This is available when you export copies of your photos, and when you create output in the Print, Slideshow and Web modules. (You can’t watermark your master photos.)
Below is a video tutorial on using the watermark editor to create, save and apply watermarks. This video is one of 55 videos from my Lightroom 4: The Fundamentals & Beyond video series. It is applicable to all versions of Lightroom starting with Lightroom 3 – including Lightroom CC and 6. (Update: My newer Lightroom CC/6 and 5: The Fundamentals & Beyond video series is now available.)
For highest quality playback of the tutorial below, once you hit Play, click on the sprocket wheel in the bottom right and choose 720/HD.