Hy the best is for windows but you may use in linux with Wine ... run really very well with wine.
I know that are excellent image viewers for linux but for use toghether with gimp there is not match, because xnview is much more then a image viewer as example may open almost any graphic format ,may do a lot of batch operations, and may run most of Photoshop plugin.
Alas Xnview is not OpenSource ...but is free and rocks
Take care to install the full package (scroll the download page )
http://www.xnview.comPS you will see also a linux version of Xnview...FORGOT IT,(its really crappy if confronted , really delusive) you need the Windows version , Wine...and for the best integration RobA "ShellOut" script