What theme are you using?
I could be wrong, but I believe that ZenPhoto will only use image_use_longest_side when using a theme function such as `getAlbumThumb`/`getNextImageThumb`/`getPrevImageThumb`/`getImageThumb` but if the theme uses any of the `getCustom`___ functions, that function can specify how to crop the image, so in that case, I don't think that ZenPhoto would use `image_use_longest_side`.