I'll be honest... mannequin heads kinda creep me out. I think the idea of using an ear sketch on a card or something is really good. Otherwise, I think maybe using something else to show scale might be good if you really don't want to freak people out.
P.S. Make sure you don't grab a copyrighted ear picture! Don't just grab anything off of the internet. Look for images that are under the Creative Commons license and then verrify that the license type allows you to use the image. (I think you can still filter google image searchess by creative license, and I know you can filter Flickr image searches by license).
You can avoid this altogether by buying a cheap stock image from somewhere like
http://shutterstock.com (for photos) or
http://vectorstock.com (for drawings and graphics).
That way you know you're not stealing someones work.