Hello John and welcome to the forum.
In most cases, pictures / images are what they are and by that I mean, depending on the quality of the picture itself your ability to resize the photo without it resulting in pixilation and a general blurring of the photograph is locked to the actual size / quality of the photograph.
That being said, for the purposes of viewing, I believe that most photo viewers will respond to....
Holding the control key on your keyboard and then rotating the wheel on your mouse to make it larger and or smaller