Well, the photos embedded in the Description section, i.e. the ones you selected as Image 1 - 24 on the lister or those you manually inserted in the description, should update immediately.
The image used as the Gallery/Header is another problem. That one needs refresh at ebay using either the Revise Listing or the new "Change Your Item's Gallery Image".
http://cgi5.ebay.com/ws1/eBayISAPI.dll?DisplayGalleryImagePageV3
Danno