Exclude post thumbnails by CSS classes

[seyfro]

As far as I understand, css class exclusions only work for images in content. It would be great if those would also be applied to post thumbnails, as I set a custom CSS class for the first post thumbnail in the loop (above fold) to be excluded from lazy loading.
This worked perfectly with a3 lazy load plugin, but unfortunately not with yours (which I like better due to being able to use low resolution preview pictures and because real image dimensions are used for placeholder images)