Hi Sharon - I have never had them create visual puns - per se. In the past I had them illustrate items such as "banana split" with magazine pictures. The best one was Princess Di and a toilet - you got it! "Royal Flush"! This was several years ago, and I don't know why I haven't done more with it. I'll bet the kids would love it!
One created a drawing that's a visual pun, though neither she nor I realized it until after she'd completed it.
She started with a picture (cut in half) of a dog, and the idea was to distort the other half in some weird way. As she worked on it, she kept saying that it was turning into a "ball" for some reason, and drew it that way.
As I was mounting it, getting ready to display it for Parents Weekend, I started laughing, realizing that she'd inadvertently created a "bassetball"!
Have any of you developed intentional (ha) lessons that challenge students to create humorous artwork that serve as visual puns?