This is actually a very complex and time-consuming sort of project. You are going to need to study AJAX very intensively and learn how to use Javascript asynchronously with PHP (or whichever server side interpreter you're using) to locate the appropriate images, determine the type and size of the image, and then replace the appropriate items within the DOM (document object model) with the selected item.
I would honestly estimate that this would probably take a beginner anywhere between six and eighteen months to put together from scratch (assuming that there are no pre-built scripts out there that can do this for you), and that it would take an experienced programmer at least 80 hours (possibly three times that, depending on the complexity of the images you will be swapping out).
If you want to build it yourself, I would follow these steps:
1) If you are not familiar with PHP and the way it handles GET requests, its image manipulation functions and its file stream functions, begin by looking for some good tutorials on those subjects
2) If you are not familiar with the DOM and the way Javascript can manipulate it (adding new elements to a page, replacing existing elements with new elements, etc.), look for some good tutorials on that subject. I wrote a five-part tutorial for HTMLCenter on the subject, so you may be able to glean some information from that.
3) Finally, if you are not familiar with the way AJAX works, visit W3Schools and read their tutorial on using AJAX.