SETTING desired youtube thumbnails

im curious if anyone figured a way to set their desired youtube thumbnail?

i read that if you are a YT partner, you can set whatever thumbnail you want . i cant seem find any info on the big G on how to set the thumbnail that i want for my vids so does anyone know the algorithm or how to set the desired thumbnail/???

I read, that out of the 3 thumbnails you get to choose from, the middle one is the EXACT middle of your video to the frame. So some people have used this to put the exact thumbnail at the exact middle of the video.
If you want to add a picture that is not in your video - as some people have done, you have to be a partner, no other way around it

Its NOT the exact MIDDLE anymore! Its not randomly either.... To me the middle image is around 66%- 69% of the video. If you using sexy videos the best type of ones are ass shaking ones because you can edit them to easily have the ass in front of the camera upclose.