Issues with stretched images in IE

I’m using Graph Paper Press’s Widescreen Theme, and am having issues with images on my slideshow template stretching in IE. I have put in a request to their support, but am getting no satisfactory responses, so thought I’d try here. Website is https://www.mayuphotography.com

I’ve got the following in my Slideshow PHP, that produces small images in all browsers:

<div class="slideshow">
<?php
$args = array(
'order'          => 'ASC',
'orderby' 	 => 'menu_order',
'post_type'      => 'attachment',
'post_parent'    => $post->ID,
'post_mime_type' => 'image',
'post_status'    => null,
'numberposts'    => -1,
'size' => 'medium',
	);
$attachments = get_posts($args);
if ($attachments) {
foreach ($attachments as $attachment) {
echo "<div class='next'>";
echo wp_get_attachment_image($attachment->ID, 'medium', false, false);
$description = $attachment->post_content;
if (isset($description)) { echo '<p class="caption">'.$description.'</p>';}
echo "</div>";
			}
			}
			?>
			</div>

If I change to from medium to full size in the code, the it works perfectly in Safari etc, but stretches vertically in IE7 & 8.

The CSS I think is relevant is:
.attachment-full,.attachment-medium,.attachment-large { max-width:760px; height:auto; overflow:hidden; display:inline; text-align:left; text-align:left; float:left; }

I’m completely self taught so really not sure what’s going wrong, so if there’s something obvious, or missing, I’d really appreciate a point in the right direction!

Thanks,

Mardi