If you edit a post and change or delete the image that EasyIndex has already selected, the original selection is cleared and EasyIndex will re-scan the post (and re-create the thumbnail if necessary) the next time the post is shown on an index. Actually this happens every time you edit a post, since EasyIndex can't tell if you've added a more suitable image without re-scanning. This won't noticeably affect your site speed - the scan only happens once after a change.