Here is the code for image sizes on secoora.org:

sizes = {'large'   : (768, 768),
         'preview' : (400, 400),
         'mini'    : (200, 200),
         'thumb'   : (128, 128),
         'tile'    :  (64, 64),
         'icon'    :  (32, 32),
         'listing' :  (16, 16),
        },

What this means is that you can take any of the dictionary keys above (large, preview, mini, thumb, file, icon, listing) and use them as suffixes to "/image_" to designate skinning for an image object.

The dimension values are the *maximum* for height or width. The aspect ratio is unchanged. The image will never be enlarged to a maximum dimension, only shrunk.