How to process arbitrary images via drupal imachecache?

Question

Drupal

I want to load high resolution background images after the page is loaded and rendered, so that optimized low resolution images for element backgrounds are loaded first using a generated request to Imagecache, and then a higher resolution image is also requested through Imagecache.
Is there a way to request arbitrary images (not from the downloads folder) via imagecache?
How can I make an animation of changing the background image of an object so that the new image appears on top?
It is not difficult to make the old image go into transparency and a new one appear instead, but how to make sure that the old one does not disappear first?