I am seeing these errors in my logs as well:
213.127.158.226 - - [27/May/2014:14:00:07 +0200] "HEAD /wp-content/gallery/House/Sneeuwklokjestraat/SN_W3_Reb/thumbs/[email protected] HTTP/1.1" 404 379
213.127.158.226 - - [27/May/2014:14:02:25 +0200] "HEAD /wp-content/uploads/2014/05/[email protected] HTTP/1.1" 404 379
213.127.158.226 - - [27/May/2014:14:02:25 +0200] "HEAD /wp-includes/images/smilies/[email protected] HTTP/1.1" 404 380
It is a bit annoying, as it clogs up my logs. Switching retina support off in Customizr options gets rid of these errors.
Please be advised that I am running NextGen Gallery as well, and my images have been imported from my other site.
Recreating thumbnails, through the Regenerate Thumbnails plugin, did not solve the issue.
Any ideas?