I really like your original thought of using a Forum activity. The forum would allow students to post and exchange ideas by posting their guesses on what is being depicted in the paintings. Of the various forum types to use, I feel that the Question and Answer forum would work for this type of activity. What is nice about QA forums is that the students cannot see other students post to a teacher question until they post their first guess. That way students are not reading all the posts before they give their guess (that would be an unfair advantage).
As a teacher, for each painting you would post a question one for each painting for a total of 5 separate posts (question). Then the students could their guess to a painting and then look at everyone elses posts and reply what they thought of the other students guesses. To encourage student engagement, you could also give the students permission to rate each other's posts with a customized scale similar to how users rate these posts. Note the instructions for this are found in the Forum FAQ.
Of course, forums are asynchronous, if what it to be more synchronous you could use a Chat activity. However chats do not allow students time to reflect on the painting or the feature of ratings.