Label 59 is a free, web app that lets you annotate photos with interactive labels. It's easy to use and actually pretty fun.
It's easy to use. Add your photo, add the markers for your labels, add some features, add actions for each marker and then publish it.
When you are done and publish your work, you get a direct link to share with people. You can also download a Zip folder with all the related files in the premium plan.
This is a great resource for teachers to use to create learning materials and for students to use for projects. Students can use a photo of something they are learning about and then add interactive labels based on the content. A Physics student could use a photo of a roller coaster and then add labels explaining the physics of different parts and features of the ride.
Check it out: http://www.label59.com/