There is a version of the FastStone Image Viewer available for download from its official website. The package is distributed as portable. The archive features an already installed version of the image viewer, but configured to run without installation.
Download package: http://www.faststone.org/FSViewerDownload.htm
Note: Click the Portable button to initiate the download.
You will need WinRAR or 7-Zip to extract the contents of the archive.