Codex query, thumbnail image

Wonder if anyone can point me in the right direction.

I have to create a page, like a case study, where I want to display, say four thumbnail images and one large image;

I plan to upload the thumbnails and display them in a row of small dive tags, these will be imported through custom fields.
Im fine with this; but any suggestion on what the code might look like, to have the thumnail clicked, but then a larger version siplayed in a big dive tag positioned on the page.

The thumbnails will be just a scaled version of the large image.

Hope this is making sense. I dont particuyalrly want to use a plugin

Any Ideas?

Any help really appreciated

Cheers