I would recommend against this for the following reasons.
PS is a resource intensive program. If all you wish to do is quickly view an image there are much quicker ways to do this.
PS will not open multiple, selected files at one time unless they are opened from within PS.
If you use something like the default Windows Picture and Fax Viewer, you can open one image file from within a folder and then view more image files in that folder by simply clicking the forward and back buttons in WP&FV.
If you have a newer version of PhotoShop, you can use the Adobe Bridge program which is an outstanding way to view and organize image files on the computer, however this also uses a lot of resources.
If this hasn't deterred you, right click the file type's you want to associate with PS and select Open With. Select PS from the menu or click Choose Program and click the Browse Button. Browse to C:\Program Files\Adobe\Photoshop and select Photoshop.exe. Place a check mark in the "Always use the selected program to open this kind of file" option and click OK.