Dave, try holding down Ctrl key and hit + to enlarge or - to reduce.
If you hit Ctrl and 0 you will return to normal size after zooming in/out as Keith says.
Also, sometimes your browser may automatically re-size some large images to fit your screen when loading the page. if you move the mouse pointer over those pictures you will see a little box/arrows type icon appear. If you see that and click on it it will re-size the image to full size. you would also be able to shrink it back down the same way.
