Image doesn't load via drag and drop
Open an image with Ristretto and then close it with Ctrl+W, so close the image but not the application. Then drag and drop an image into Ristretto for example from the desktop to the Ristretto window. The image will not be loaded and only the thumbnail bar will be loaded.
However drag and drop works for empty Ristretto sessions when Ristretto won't load an image on startup.