Why start with an image? Because it’s less restrictive. It opens your imagination; words tend to close it.
Here’s another reason to start with an image: an image has built-in emotion – if you choose the right image. Fiction is all about emotion. No emotion? You’ve got nothing. Your idea, no matter how wonderful, will fizzle out. Or you’ll have a bunch of weird emotions tumbling around, which you can’t get a handle on… and the novel or short story fizzles out.
You may get an idea for a story. Let’s say that we’re writing New Adult fiction. Our heroine falls in love with someone she’s only “met” on Facebook. Unfortunately, she’s fallen in love with someone who doesn’t exist. Someone created a fake profile, specifically to lure her into a trap.