There is a huge number of different image formats and it is very frustrating
to receive an image that cannot be opened.
XnView
can view around 400 different kinds of image and convert into 50 different
formats. It is a good way of browsing images on a computer with a capable
explorer window that can show previews of images if required.
It is not just formats that can be changed as XnView can be used to rename
batches of images.
XnView is also available for a wide range of operating systems apart from
Windows, including MacOS X and Linux. A
full
list is on the XnView website