*Gulp* guilty, I write this way. I start a beat outline by inserting the scenes I have 'seen' first then fill in the other beats to get to those scenes.
I've heard the first step to recovery is to admit I have a problem. ;) (Imagery-aholics Anonymous, Michigan Chapter)

I used to write this way, especially working with random phrases and building out from them when I wrote short stories. It actually lends itself to the short story format so it wasn't a problem (hence my flagging membership in Phrase-aholics, the sister org to Imagery-aholics). Having studied screenplays for three years, all that went out the window pretty early on into the process. I think I've started all my screenplays with a concept, then bounce between developing the plot and developing the characters. I've tried concept>plot>character, but some of my worst stuff was written that way. Retrofitting characters into a plot just doesn't work. You really feel the hand of the writer. Now, I develop plot and character concurrently and that seems to be working well.

@tracinell: I don't think there's necessarily anything wrong with 3x5 index cards. Evidently it works for Chaon. And I've been in countless writers offices, especially TV, in which it seems like entire walls are covered with index cards as part of the story development process.

Whatever it takes. That's all that matters. If index cards work for you, awesome. If not, toss them.

There's no right way to write.

Me? I use 3x5 index cards for plotting. After I brainstorm as many scenes and beats as I can, I transcribe them to cards, then work through a variety of orders, shuffling and reshuffling them until I feel like they offer a pretty coherent take on the plot. Then I move that to an outline.

One really helpful aspect of cards: You can divide up scenes and beats into three stacks: Act One, Act Two, and Act Three. I don't know about you, but I can pretty much look at a scene and know which act it belongs in. That separating process can be a helpful way to start wrangling your plot.

I do write that way. I always have an image or series of images first. It's why I wrote poetry for so long, because you can write directly from an image and play with it, and don't need to develop anything like a plot or characters.

But my screenplay ideas still always start with images.

What seems to work best for me is to start with an idea or image. I brainstorm, note down anything that comes to mind, sketch an outline, zero in on main characters. Then I write a very first draft, mostly following the characters and letting them take me along (while keeping an eye on the basic outline I had).
Now I know my characters better, and my story has more of a shape, I sort out the structure, make it more defined.
I check the pace, rhythm, feel.
I go back to characters, make sure they are coherent throughout and real.
Then Scene work - structure
And Scene work - writing: style, description, dialogue