Thumbnails top position miscalculated

Hi!
It affects tile’s view if lazy loading of images is enabled (in Jetpack in my case but I think it’s going to happen with other plugins as well). The “top” property for rows other than the first one gets a wrong value and images overlap one another.
I believe that it is necessary to change the way to calculate the value. Now it takes size of non-existent image (it’s not loaded yet) and that’s the reason for the error.
cheers,
Jarek