Portable XnView (Win | 9.3 MB | RS)
Free graphic and photo viewer without installation.
Extract (with a PortableApps folder at the device's root) and run XnViewPortable.
If you drag'n drop a file on (or open with) XnViewPortable: it will be opened in XnView.
Integrated in shell explorer while running: right click on a picture to show thumbnail, convert, send to ImageShack…
Settings and integration of installed XnView should be preserved.