The online availability shows the original image, no matter the original size/ratio, in a 300 x 300 square.
This means that if the original image is a wide image (e.g. 900 x 300) it will look squished in the floor plan description.
However, when the user clicks that image it shows the original image in the full-screen image viewer at the original ratio and size.
What all this means is that the best size is at least 300 x 300 (although, the higher the better because the user can zoom in with the image viewer) and the best ratio is 1:1 (square).
In the end the API will return a url to the original image (original size/ratio)