What is the preferred approach for using dual images (fast-loading thumbnails and full-sized photos)?

Question

As my posterous/twitter photo blog is reportedly going the way of the PC at the end of the month, I'm going to create my own. I don't want the page to take aeons to load, so I want to use small (in file size) images for the thumbnails but then, when a visitor clicks on it to view it full-size, show the "real" image (full-size, both from a screen dimension perspective and as to its file size), in a sort of reverse bait-and-switch.

So I reckon I'll need subfolders like:

Project
    Images
        Thumbnails
        Fullsize

I imagine something like this is done a lot. Is there an accepted/"received" way to "minify" the images to use as thumbnails (some webby type tool), or will I have to save each photo at a smaller size, one by one?

Are you asking if this should be on-demand vs. cached on disk, or asking about a preferred tool for batch processing? — Mark Ransom, Apr 11 '13 at 22:25
Although an answer to the first question will probably be needed eventually, my immediate question was the latter. — B. Clay Shannon-B. Crow Raven, Apr 11 '13 at 22:29