There is a way to upload your custom thumbnail for YouTube shorts, but it does not get shown as the actual thumbnail that the viewers see.
In short, it seems the YouTube randomly selects a moment of the video as its thumbnail, which seems attractive to hook the viewers.
I wanted to replace the thumbnail to a high quality one with vibrant colours. But it does not seem to be possible for now. As mentioned in the video below, the priority should be on the thumbnail for YouTube normal videos, not the YouTube Shorts.
YouTube Shorts is a great way to expose the video/account to a lot of viewers, but what would gain a real fan/follower would likely be normal videos. (Although yes, there are YouTubers who gain many dedicated fans mostly through shorts).