If you make Canva thumbnail for Medium I always found YouTube Thumbnail to work the best because you can resize it in Medium and also you can use gif and moving animation to really spice up your work. Good luck my new Medium friend and I know you will do great things! Welcome to the Medium family and if you want to learn more things check out all the people out in Medium there are so many pros out there with knowledge ready to be shared and I hope you find more of them. 😊