To continue working on your artwork elsewhere—a photo editor, a game engine, a viewer—choose File > Open With Shift+Command+O.
Dottie writes a flattened PNG of the canvas—the stacked layers in Composite mode, or the tileset sheet / Wrap Tiles group in Tileset mode—and lists every installed app that can open PNG files. Choose one and the image opens there immediately, leaving your Dottie document untouched.