@skynet: Add the following CSS:
a:focus img[class*="align"], a:hover img[class*="align"], a:active img[class*="align"], a:focus img[class*="wp-image-"], a:hover img[class*="wp-image-"], a:active img[class*="wp-image-"], #content .gallery .gallery-icon a:focus img, #content .gallery .gallery-icon a:hover img, #content .gallery .gallery-icon a:active img, a:hover img.wp-post-image, a:focus img.wp-post-image, .gallery-thumb a:hover img.attachment-thumbnail, .gallery-thumb a:focus img.attachment-thumbnail {
background: none;
}
Regards,
Mahesh