I found it by accident, although I searched all over net so many times: how to enable custom youtube thumbnail without being partner.

Here is how:

Step 1

Sign in in your CHANNEL account and click on the arrow in far right corner to activate drop down menu.

Step 2

Choose YouTube Settings from the drop down menu.

Step 3

You will get the Overview page where you can set up some parameters for your channel, but now you should click on the VIEW ADDITIONAL features link.

Step 4

On the FEATURES page you will discover VERIFY button. You should of course click on it : )

Step 5

You need to verify your account via mobile phone. You check the button “text me the verification” code , enter your mobile phone number and you will receive code via text message.

Step 6

Enter the six digit code you received in the blank box, and click SUBMIT button.

Step 7

You have verified your account and you can continue to your channel.

I suggest you go to video manager and edit your uploaded videos

You will notice new button  that enables upload of the CUSTOMISED THUMBNAIL image for your videos

Enjoy!