Main thumbnail size is your series cover
Episode thumbnail size is exactly what it said, your episode's thumbnail
![](https://forums.tapas.io/uploads/default/original/3X/9/4/940a7ddae8aa05cace3e913494fd3d7205612235.png)
Panel size is actually the size of a page you can upload, not the actual panel itself. The max file size is 800 x 1280 for webtoon and they all have to be in jpg format!