From XnView Wiki
If you double-click on a file on your desktop that ends in ".doc," MS Word is started and your document is opened. This happens because Windows recognizes that MS Word is the program to use when opening ".doc" files. This relationship between file types and the programs used to open them is accomplished by a table in the Windows registry which keeps track of these "associations."
File types are defined by the file's extension, which is the part of the filename to the right of the last dot in the filename. The extension for the file "image.jpg" is ".jpg," and ".jpg" in turn is the file type that Windows looks at when determining which program to use when opening .jpg files. Because Window identifies file types by that last part of the filename, you should never change a file's extension unless you explicitly need and want to do so—and know what you're doing.
It is possible to associate XnView with any or all of the file types that XnView is capable of opening, which is essentially all image files. By setting up file associations, in other words, you can have XnView be the program that opens and displays the image when double-clicking on any image type. Most common image types (BMP, JPG, PNG, GIF, etc.) are associated by default with the Windows Picture and Fax Viewer, a nice image viewer program that comes with Windows. That viewer, however, is very simple and limited compared to XnView, so you may want to change your associations.
Important: Although Windows provides a way to associate programs and file types (in Windows Explorer > Tools > Folders > File Types), this method will fail when associating files with XnView. When associating image files for XnView you MUST use XnView to set the associations.
- Start XnView
- Go to Tools > Options > System Integration > Associations
- Individually select the file types that you would like to associate with XnView, or use the tools provided to select all or a subset of all file types
- Confirm the options dialog by clicking the "OK" button.
If associations are not set properly, XnView may start up when double-clicking on a file but may display an error message right after starting: "Error opening <path name>". In this case please re-associate the image files using the above steps.