A good sketch can be developed from pretty much anything that enters your mind. All you need is a good starting point. Try this: clip photos out of magazines, newspapers, or from websites that show people in interesting settings — playing touch football, walking the dog, on a date in a restaurant. Use these as your starting point for a sketch. That's the scene the audience sees as the stage lights turn on or the video fades up.
Let's say you have a photo of four older men sitting at a table drinking beer. Ask yourself questions like: Why are they together? Are they all brothers? Are they the last survivors of the Hindenberg disaster? Are they old army buddies meeting for a reunion? Are they the four ex-husbands of a woman whose funeral they just attended? Are they actors filming a commercial for some embarrassing medical product? It's an incredibly simple starting point, but the possibilities are endless if you ask yourself the right questions.
Brainstorm an idea using one of your photos and write a short sketch based upon the idea. Sometimes all it takes is a gentle nudge to get you started on the road to a great sketch.