Unless the images are created dynamically, that technique seems pointless.
The images are being created by the site designer (rather than dynamically by the site itself), so why not simply use them in the markup in the first place?
If images are disabled, the alt text will be used.
Alt text is not significantly less accessible than regular text. (Some browsers support the styling of alt text using CSS, which would mean that alt text is identically accessible compared to regular text.)
From what I can tell, there's absolutely no benefit to using js/jQ to dynamically swap in images which already exist within the site directories.