Links to full-size images

I use IImageBrowser to put thumbnail images into my posts. I want to set up my images as follows:

thumbnail (200×133) in post linked to ‘snapshot’ (640×480) with an option to download the full-sized image (can be up to 1178×960 or something like that; it’s whatever my digital camera puts out or whatever I scan the images at). The idea is to give people the opportunity to quickly download the inline thumbnails, then look at a larger pic that still fits on their screen, and, if they want to get a hi-res image to print, they can download that.

I would ideally like to have this process have the same look and feel as the rest of the blog – i.e. use the same template to put all the decorations and widgets in. I really want two navigation buttons – “back to post” and “view/download fullsize image”.

Any plugins out there to do that or something similar so I can use for a template to hack (I guess I need to learn PHP first…..)