You could have the characters in the pictures carry on a conversation. You could write the conversation in speech bubbles and have the students write the conversation correctly. This would cover capitalization, comma use, quotation marks and many other things - especially if you threw in things like book titles!
Speech Bubble 1: What are you looking for?
Speech Bubble 2: My umbrella.
Speech Bubble 3: Why?
Speech Bubble 4: Do you hear the rain?
Henry asked, "What are you looking for?"
James answered, "My umbrella."
Henry asked, "Why?"
James answered, "Do you hear the rain?"
You could also reverse this where you give the conversation and the students write what belongs in the bubbles.
Students could try to see how many words they could use before repeating a "speech" word - said, screamed, answered, yelled, whispered, croaked, gasped, sighed, etc.
This came to mind because a fellow teacher and I just gathered information to present a "Say What?!" workshop at school all based on speech bubbles and cartoons. Many of the ideas we had could be adapted to what your doing. Of course, that's all at school and we're out.
If you want me to send you some of the ideas we had, let me know and I'll send them to you when we're back in school or over the summer if I ever make it up there.