From HumorTimes.com:

Always wanted to be a cartoonist, but can’t draw worth a darn? Test your abilities by writing the caption for a pre-drawn cartoon! Just send in your idea for the cartoon below to Humor Times Caption Contest! Win a Free Subscription to the Humor Times hard-copy edition!

Here’s the blank cartoon they provide, courtesy of Daryl Cagle:

Here are two of my contributions: