When you look at a list of picture files in Windows Explorer file management program, you can often double click on a file name and the picture automatically displays within a program. When you double-clicked on that file, you launched a program because that file format was associated with that kind of program. Windows often comes with a number of file format associations that are preset by default.
There are plenty of files, especially picture files, that have not been associated with any program. If you want to know what kind of program is currently associated with a certain kind of image file, right-click the file name and then on Properties, which should list the file association, if any.
You may prefer to associate a different program with a particular kind of picture file, rather than the program that is automatically chosen by Windows, such as Imaging for Windows Preview, a program that comes with Windows. Some programs make it easy for you to reset file associations — they include it as a menu item in the software.
In Windows Explorer or My Computer, display the file you want to make or change an association for. Right-click the file name, choose Open With and Choose Program, then click a program to associate with this file. If you want this program to always open this type of file, also choose Always Use the Selected Program to Open This Kind of File. When you check this option, the program you selected will now automatically open this type of picture file.