Hello, I'm an utter amateur when it comes to coding anything and I feel over my head right now. Anyways, I'll describe what I'd like this page to do.

I want to be able to click on an image, and have it replace an image elsewhere on the page.
I plan to have multiple (20+) image thumbnails, that when clicked will replace a main image with a larger version of the thumbnail.
I've been trying to accomplish this with OnClick, Radio Buttons and the like and I just can't seem to figure it out.