Note that both have an important flaw of their own, so you may want to read this post. And for those who've forgotten (somehow) the differences between the two, I've also described how each fullview work(ed/s).
Old image fullview.
Large images in posts and private messages are shrunk to about 400px, and given a thick grey border. When you click such images, the page is hidden, and the full picture is shown. If it's bigger than your screen, it gets clipped, and you can scroll to view it. You could scroll using the scrollbars on the window or by clicking in your mouse wheel and moving the cursor. When you're done, you would click the image to get back to the page.
This, unfortunately, had a few disadvantages, especially in Turtle. Turtle introduced a bug where a thin strip of space (anchored near the right edge of the window) would act as a small "invisible wall" for the mouse. If it covered up an element and you tried to click on that element, nothing would happen. The bug is still here (though it's about to be fixed, now that the old system isn't in use), and can be extremely annoying when trying to interact with textboxes, links, etc..
New image fullview.
A large image is shrunk and bordered like before. Clicking it opens it in a fancy modal dialog (resembling those that appear when interacting with the buttons on the BBCode Console while posting); this modal stretches to mostly fit your screen. The scrollbars are on the modal, such that its buttons are always visible.
The modal has "Previous", "Close", and "Next" buttons. You can use the "Previous" and "Next" buttons to "travel" between all fullviewable images on the page. The modal can be closed by clicking the image or by clicking "Close". As soon as a certain script update is added to the site, there'll be another button that allows you to make the image shrink to fit the modal (and hence the window). This turns the fullview modal into something like a "middleview": the image is larger than before, but small enough to still fit in the window.
This script has the disadvantage that you cannot scroll by clicking in the mouse wheel and moving the cursor; this scrolls the page behind the modal. You must use the scrollbars.
If a good number of people have voted in favor of the old system, I'll, with Dev's permission, give him the files he needs to revert the site back to that.
None.