Yes, you can do this from the left menu under Create > Custom Buttons/Images.

Click "New Custom Button", enter the URL to image, check the boxes for all the areas you want the button/image to be available, and save.

When you go to select the Buy It Now button/image (or whichever image area you selected your custom button/image to be available) it will appear in the list of images to select, along with the stock images.

Keywords: upload, s3, picture