Is
https://static-cdn.jtvnw.net/previews-ttv/live_user_cohhcarnage-440x248.jpg
440/248 = 1.77
Self Disco.
I misread your post and give game art information.
But 1920x1080 will work for stream thumbs. Since thats what people stream at
https://static-cdn.jtvnw.net/previews-ttv/live_user_cohhcarnage-1920x1080.jpg
ratio 1.7777777*