question mark and characters at the end of image file path

I have a question mark and 6 characters at the end of all my image file names its the

?3d87c0

at the end of

http://bitcoin-betting-guide.com/wp-content/uploads/2013/12/nitrogen-sports-logo-42.jpg?3d87c0

Seems to be in the browser as it does not show up in the file path WP media library or on the server. Could this have something to do with EWWWW?

Thanks

https://wordpress.org/plugins/ewww-image-optimizer/