It depends on the photos you're referring to... The images uploaded to the eBay gallery aren't removable, they will remain visible for 90 days, at which point the listing is no longer stored on eBay and can't be viewed at all.
Images the seller puts into the description themselves are hosted on other sites (eg Photobucket), so they can be removed from those sites at any time, and they will disappear from the listing (if the HTML remains in the listing, you'll usually see a notice from the third party site saying that the image has been deleted).
You can upload images into an eBay gallery that are hosted on third party sites, so theoretically if they are deleted from the other site, they wouldn't be visible on eBay anymore - I say theoretically because I upload all my images directly to eBay's gallery so have never tested out the third-party hosting.
eBay also provide a snapshot version of listings for anyone who has purchased it, which is supposed to include any uploaded photos, but if they are deleted I suspect it would just be everyone else who can no longer see them.
