Joseph,
I think it isn't actually an Outlook setting you need to change. I believe that Outlook will just use the default program to open a certain file extension. If you try this first. Open the picture from Outlook, see what program opens for it. Then save the attachment to My Documents (or other) and then double click it there. If they both open in the same program then here is something to try.
From the file in My Documents (or other), right click and choose Open With. This will then give you a list of programs. Choose the one you wish to use and tick the box that says always open file of this type with this program. Then OK. Then go and test it in Outlook. Double click the attachment and hopefully it'll open in the program you have specified for that file type.
