Next / Previous Post using Post Thumbnail?

I am using this code on my blog for the moment:

<div id="cooler-nav" class="navigation">
                <?php $prevPost = get_previous_post(true);
					if($prevPost) {?>
					<div class="nav-box previous" style="float:left;">
                    <?php $prevthumbnail = get_the_post_thumbnail($prevPost->ID, array(100,100) );}?>
                    <?php previous_post_link('%link',"$prevthumbnail  %title", TRUE); ?>
					</div>

                <?php $nextPost = get_next_post(true);
					if($nextPost) { ?>
                <div class="nav-box next" style="float:right;">
					<?php $nextthumbnail = get_the_post_thumbnail($nextPost->ID, array(100,100) ); } ?>
					<?php next_post_link('%link',"$nextthumbnail  %title", TRUE); ?>
                    </div>
                </div><!--#cooler-nav div -->

The result can be found on this page: https://funny-pictures-blog.com/2011/03/24/in-soviet-russia-pictures/

The problem is that it does not work with videos. so if a video is dispalyed on the blog post only, it ONLY shows the next video post, and that can be posted a few weeks before. Is there any code that i can use that shows both videos and thumbnails. so if a video post have no image, it will use a default thumbnail or something like that.

An example on how a video post looks like: https://funny-pictures-blog.com/2011/03/24/funny-videos-funny-bird/