Well, you can manually change the image and with this, do a little workaround by changing the dimensions (like stretch the image with Photoshop or change dimension 1920x1200 file to 1440x900). I am unaware you can choose Fit Screen, Stretch and the others. Simply navigate to /system/library/coreservices/defaultdesktop.jpg and use what ever image you want. Be sure to rename the new image to defaultdesktop.jpg and replace. You'll need to authenticate with your username and password.
I change mine too. 😎