Fotobabble is a free service that allows users to add audio (voice-over)
to any uploaded picture. Users then receive a link (that can be shared with others) that leads to the saved picture . Easy to use, there is no freeware to download. You just need internet connection and a working microphone.
An app for phones is also available. 

Here's a full tutorial from Fotobabble:

Here's a student example:

Here's a teacher sample: