@livyfairbairn: I'm not seeing this on the front of either http://heyitsmetheblog.wordpress.com or http://firstaidfashion.wordpress.com. Are you talking about a different blog?
@jacquelinejc: You're misusing the format. The gallery format is used to indicate that a post is or contains a gallery of images. Your May 19th post is set as a gallery format post but it doesn't actually contain a gallery: it's a regular post with a couple of independently inserted images. When the post doesn't contain a gallery but is set as a gallery format post, the note on the front that says "This gallery contains [n] photos" refers to the number of images that have been uploaded from your computer via the Add Media tool of that post, and the thumbnail will be one of these images. Both of these are independent of what images you have inserted in the post or of what images you delete from the post. If you don't want the wrong thumbnail on the blog front, then (assuming you're not using this image in another post), you need to go to Media > Library and delete it.