If it's only to watch a picture (aka "preview a picture"
) Preview is more than enough. It even allows you to save it to a different format, rotate it, etc.
Running Photoshop everytime you want to watch a pic is a waste of memory in my opinion...and a waste of time if you start Photoshop every time you watch a pic.
Anyway, if you want to use PS for your images, right-click on an image, click "Get info", choose PS in the "Open with"-box and click on the "Change all" button just below. You do that once with every filetype you want to open with PS, and all of them will open in PS per default.