Ristretto is described as 'The Ristretto Image Viewer is an application that can be used to view, and scroll through images' and is an app in the Photos & Graphics category. There are more than 25 alternatives to Ristretto for a variety of platforms, including Windows, Linux, Mac, Xfce and PortableApps.com. The best alternative is IrfanView. It's not free, so if you're looking for a free alternative, you could try nomacs or ImageGlass. Other great apps like Ristretto are XnView (Free Personal), JPEGView (Free, Open Source), Preview (Free) and gThumb (Free, Open Source).
nomacs is a small, fast image viewer able to handle the most common image formats including RAW images. Additionally it is possible to synchronize multiple viewers. A synchronization of viewers running on the same computer or via LAN is possible.
ImageGlass is a simple picture viewer with which you will be able to display your image collection in a very similar way to the one that Windows provides by default, but with some additional functions.
gThumb is an image viewer and browser for the GNOME Desktop. Features Image Viewer * View single images (including GIF animations). Supported image types are: BMP, JPEG, GIF, PNG, TIFF, ICO, XPM. * View EXIF data attached to JPEG images.
PhotoQt is a simple, yet powerful and good looking image viewer, written in Qt, published as open-source, and completely free. Photo comes with a variety of features. Most features you would expect from a image viewer should be there.
QuickViewer is a graphic image viewer for comfortable browsing of many images. This software performs reading of image data in advance. And drawing the imported image data on the screen with OpenGL. You can browse images with much smoother response than ever before.