What I have learned so far is that Voice Thread is great for adding voice and video voice clips over pictures. It also has an embed code on it so you can easily post it to your website. The first three slide shows are free and then there is a minimal cost for an educator license and an option for larger school licenses.
Voice Thread also has the option of letting people comment on the slide shows themselves. There is a moderator option, which could add even more options for student interaction with slide shows.