There are two ways to associate image formats with Handy Viewer.

1.The first one: when you install Handy Viewer, on the final page of installation Wizard you will see the option "Associate image formats with Handy Viewer". It is enabled by default. So please keep it enabled, and the installer will associate file extensions automatically.

2.The second way. Run Handy Viewer, and select in the main menu: File -> Set File Associations... Confirm the operation in the dialog box.