This has puzzled me for years: How does Blender decide the window where it will display the rendered image when I hit F12 to render?
By default it picks the largest area, unless an image editor already exists.
You can alter this behavior by setting the Display option in the render settings:
Does not change the UI. The render will not be automatically shown.
Opens a separate window containing only an image editor showing the render.
Image Editor (default)
Renders into an image editor. Turns the largest area into an image editor if one doesn't exist.
Replaces the UI with a full screen image editor showing the render.