Exif wMarker is a free software to add Text based watermark to your images. It can add watermark to multiple images together in a batch. It is extremely easy to use, and provide lot of customization options for the watermark to be added. You can control the font, size, position. You can also specify opacity, and special effects. Exif wMarker also lets you add EXIF and IPTC information to images.
The best part of Exif wMarker is its simplicity of use. You just select the images that you want to watermark, and then specify the watermark information. Exif wMarker will take care of the rest. Exif wMarker can also resize your images after adding watermark to them.
Exif wMarker is a good tool to add description to your images in a slide show. Another good option that you can try is PicMarkr. This is an online tool to add watermark to your images.