Most screenshot tools want a subscription or an account. Artboard Studio is free and open source, and runs entirely in your browser. Lay out artboards, drop in device mockups (iPhone, Android, tablet, desktop, or your own frames), place your screenshots, add text and shapes, then export PNGs at the exact sizes Google Play and the App Store require. Projects save to your browser's IndexedDB, so nothing leaves your machine and there's no sign-up.
