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:
Sign in in your CHANNEL account and click on the arrow in far right corner to activate drop down menu.
Choose YouTube Settings from the drop down menu.
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.
On the FEATURES page you will discover VERIFY button. You should of course click on it : )
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.
Enter the six digit code you received in the blank box, and click SUBMIT button.
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