iWatermark – This one if probably my favorite of the bunch. It is easy to use and supports text watermarks as well as images (logos). It is only available for iOS so android users will need to check out another option. Cost: $0.99
Impression – Another straight forward and easy to use option. I find the text options on this one to be a bit better than others. Again another iOS only option Cost: $1.99
Add Watermark – Best Android option that I was able to find lots of features and fairly easy to use. Less buggy than any of the other Android based options I played with. Cost: $1.99
Regardless of the tool you use, it is important that you use it. Protect your images and generate interest in your content by making sure you are getting noticed for your work.