Prompt:

beautiful smiling maiden, long flowing hair, art nouveau poster, art nouveau style border treatment, by Alphonse Mucha --ar 2:3

Notes:

Since this uses the word “poster” in the prompt, I set the aspect ratio at 2:3 (—ar 2:3). Otherwise, Midjourney would add white space to the sides of the square image. To get around that problem you can add “square” to the prompt when using the default aspect ratio.

Settings:

Beta Upscale --v 4 --q 2 --ar 2:3