Hi, It's clear question and it's important to know that ,because my video's image looks like shit, BTW i was put custom thumbnail image in the past by (monetization) option but now YT make it more complicated to make partnership! It will be nice if there are any further method ! THANKS HAVE GOOD DAY
You can't, you have the monetization turned on for the channel. After that you can put custom thumbnail even if you dont monetize those videos. There is no other way around it. I forgot how the spliting for the 3 suggested thumbnails was done but if you insert the thumbnail for a sec or 2 at the right time you can get it. You can try that
What I would do is put your thumbnail image at the end of the video for like 60 seconds then it will be available as a screen shot. You can also use that time to put up some annotations to other videos or tell them to click the link in the description.
