Right click on a picture file and select properties. It will come up with that file's attributes. Where it says 'Opens with:' it will give you the name of the program that is the default setting. Next to that is a 'Change' button. Press that and then select the program you wish to use instead.
This will change the default setting for that file type. So if its a JPG you've slected then all JPGs will open with that program. You need to make the change with at least one of each file type, BMP, GIF, etc so that they all open with your selected program.