Actually, it's totally doable with good prep! Blocking the main beats first saves a ton of time. I've finished a solid two-page scene in about 25 minutes when the dialogue was really flowing.
Sam's right to be skeptical about a full scene in 30 minutes. The real trick is what you're writing for. A quick character sketch for a game session? Sure, 30 minutes. A polished chapter for a novel? Never. The image prompt probably worked because it gave you a concrete problem, like a person holding a bloody key. That's a story. A random word like "blue" isn't.