Add Entity API integration for the generated thumbnail

This issue has bothered me over a month so I finally decide to post this here.
I'm using video module to display videos and it can automatically generate a video-thumbnail using Zencorder.
In my case, I want to display the video-thumbnail image in Search API - Views. But I can neither index the video-thumbnail field nor index the video field by Search API. I simply cannot find any related fields via "ADD RELATED FIELDS" section in Search API index page.
And also, I cannot find the field in Search API - Views page.
(Besides, I can find this field in the original Views.)

I don't know if I need to do more configuration via entities, indexing or field settings somewhere.
Any advises would be great!!