Firstly, it would help if we could see your code. Either post a link to your code on a server, or include your code in your message, using the CODE tags (use the '#' icon on the message editor).
I would guess that what you need to do is to set the display of your images. For each image that you wish to display, set the style.display to "block". When you want to display another image, set the style.display of the original image to "none".
What I would do is to include all images within your DIV, with the default style.display of each set to "none". Then, each button would activate one image, setting the display.style to "block". You will need to keep tabs on any previous image, setting its display.style to "none".
An alternative (but less efficient) way would be for each button to set the innerHTML property of the DIV with the desired image.