LIghtroom 4 problem
Are you looking at the Navigator Pane at the top left of the screen?
This shows the image under the mouse pointer in the film strip at the bottom of the screen and changes as you move the mouse pointer.
If this isn't your problem, can you give some more details. Do you have the screen in Grid, Loupe, Compare or Survey mode? Which image changes? Does it change when you do nothing at all? No you have any filters turned on?
Have you initiated the Slide Show?