That is possible with CSS:
#pp-afterslider img.alignleft:hover, #pp-afterslider img.alignright:hover, #pp-afterslider img.aligncenter:hover, #pp-afterslider img.alignnone:hover, #content img.alignleft:hover, #content img.alignright:hover, #content img.aligncenter:hover, #content img.alignnone:hover, #content .wp-caption img:hover {
opacity: 0.99;
}