This is the most straight forward method to ensure every time you open your desired image files in Windows XP, your favorite image viewing application pops up with the images ready to be seen by you. For doing this, right click the icon of your image file, and select "Open with > Choose Program". You will see a list of programs in front of you, which will be categorized into two groups. On the top in the list, you will see some applications grouped as "Recommended Programs". You will also see some other applications grouped in a separate category named "Other Programs" on the bottom of the list.

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Check thoroughly if any group of applications in the list contains your favorite image viewing application that you want to make default for the particular image file format, and select it. After selecting the application icon in the list, click the checkbox before the caption "Always use the selected program to open this kind of file" right below the window pane, and click at the "OK" button. You've done it! Now all the image files pertaining to that particular file format will be opened in the application that has been selected by you as the default image viewing application for the format.
If you are unable to find the application right here in the list, don't panic. There's a lot more that can still be done. Below the caption mentioned above, you will find a button labeled "Browse". Click it, and you will reach the "Program Files" folder inside your Windows XP installation folder. By default, when you install any Windows XP application, this folder is auto-selected by the PC for you to install your application into.