Where are video thumbnails (v2.7)

Dear admins and users,

I upgrade my site to 2.7 recently.
Some of videos of my site do not have thumbnails. With the old version (2.6) when i got that ptoblem i just had to upload an image using a ftp client to the correct folder.

But with this new version i was not able to locate the folder in the server where all new thubnails are created.

when i show up the properties of the missing image i get a link like this :

http://domain_name/thumb/4ca4238a0b/1_10602.jpg

but when i conenct to the server using my ftp client i can not see the folder called "4ca4238a0b".

So guys please let me know where are they , those images ?
Is there any instruction in .htaccess which make this wired thing happen or any where else ?

What is the permission of folder thumb ? It should be 777. What happens if you login to admin area and click the "Create Thumb" link in video details page ?