Psykopaint is straight up FUN. It is photo editing meets paint and it allows for tons of creativity and artistry coupled with extreme ease of use. Let your students run wild with it. Give them each a picture from a story and let them transform it into their own artistic genre, or have them create their own original work. Be creative. Have a blast!
(Oh, as a word of warning, I not sure what kind of filter is in place in the public gallery. It seems fine, but it is just worth looking at before you send you class to work.)