This image watermarking app is excellent at what it does. It is easy to use, integrates well with the iPhones Camera Roll, and makes it easy to drag watermarks into the best position on each image.
Its best to have a few color variations of your favorite watermarks saved, so you can easily swap them if they match a particular background too closely.
This app makes watermarked sharing easy and convenient.
aegraves about iWatermark - Watermark Photos, v6.4