ThumbnailMaker
Two of the tools are free and need no account at all: a viewer that shows any YouTube video's thumbnail at real feed size, and a thumbnail text editor.
The paid part generates a finished 1280x720 thumbnail from a description of the video. What makes it different from other AI image tools is the correction loop: when a result is 90% right, you say what to change and it edits that one thing instead of regenerating from scratch and losing what worked.
Everything is sized for YouTube, including keeping the bottom-right corner clear where the duration stamp goes. Built and run by one person.