1 high contrast between foreground and background. Darkness and brightness and complementary colors are good to play around with, high stauration vs low, blurred effects vs not blurred ect.
2.focus on a few specific details, I hear faces do best for advertising. Just don't be too elaborate. Put something important front and centre make it eye-catching.
3. tapas guildines for pixels are wrong, you can go as big as 500×500, possibly bigger but that's what I've gone with.
4. If you can, gifs are very eye catching! Even if it's just a glimmer added in afterwards.
I assume this is for a series thumbnail, if these are episode thumbnails, I pick a nice looking panel in the episode and use that, other people use their volume cover. If you do the latter I'd use different color combinations for each chapter to make it easier to differentiate.
