You can use any image editor to just put Text Over the Image.

Pretty obtrusive.

Or add transparent text, which is almost the same, but could have been lighter.

Free at http://www.irfanview.com

Or use Picture Shark which blends an note, image, copyright or whatever (from a jpg you make) over the image, feathered in. I could have made it even lighter, unlike the plain text which is solid.

Can be found free at http://www.picture-shark.com