Constructing a presentation can involve multiple images or shapes on one slide. Objects are piled on top of each other in the order that they are created.
You can move items forward or back by using Draw>Order on the Drawing toolbar. (Drawing Tools> Format in 2007). However, how can you select an object if it is buried under other graphics?
PowerPoint allows you to cycle through every object on the slide by selecting one object and then using the Tab key to cycle through all of the objects on the slide. Objects can be graphics or text boxes; Shift+Tab cycles backwards through the objects.
Click on any visible object; press the Tab key until you see the selection boxes that indicate which object is selected.
Here's a Flash tutorial by Sonia Coleman. (It opens in a pop up window, so you may have to tweak your Google/MSN anti-popper toolbar) Selecting Objects