When looking at/through images, we hardly want to see a single picture. Most people expect an image-viewer to open all images within a folder.
Ristretto displays the image, nothing else (thumbnailbar only shows this one image). Controls don't do anything. If the user wants to view other images of that event/folder, he/she has to either close Ristretto and then open another image via Thunar or use the “Open Folder” menuitem/toolbar-item and navigate to the correct folder.