Trying to find a solution to this myself – I think it is that the automatic CSS coding added by the wp-includes/media.php file gives a percentage figure for thumbnail width – eg if you have 2 columns each would be 50% wide which makes it too wide in IE if any other CSS increases the width by adding borders etc – would make it more than 100% so last image drops underneath the others.
Adding the required coding to your themes style.css also works as this is called after the tinymce css files. It also avoids altering tinymce files that may be changed back again on upgrade.
Add any changes you need below the body tag in your style.css file. For example, these are the changes I needed:
body.mceContentBody {
background-color:#FFF !important;
background-image: none;
text-align: left;
}
