GPicView is a lightweight GTK+ 2.x based image viewer with following features:

* Extremely lightweight and fast with low memory usage

* Very suitable for default image viewer of desktop system

* Simple and intuitive interface

* Minimal lib dependency: Only pure GTK+ is used

* Desktop independent: Doesn’t require any specific desktop environment

GPicView is available from ubuntu repository,just search gpicview in Applications->Ubuntu Software Center

GPicView can be launch from Applications->Graphics->Image Viewer,you can set this as the default image view in its preferences window (select preferences in right click menu):