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.
Merge the “Open” and “Open Folder” menuitems/toolbar-items
Drop the “Close” and “Close all” menuitems/toolbar-items, within the folder-concept those don't make any sense any more because:
either the user wants to open another folder, then “close” is an unnecessary intermediary step
or the user wants to stop looking at images all-together, then closing the application is the correct action
To improve browsing-by-folder in Ristretto an overview/“panorama-view” could be implemented. This would basically be an icon-view/thumbnail-view that shows all images in the current folder, by double-clicking Ristretto changes back to its original layout and displays the image. View-mode toggle-buttons could be placed in the toolbar for switching between the two views (apart from menu-items or keyboard-shortcuts).