How do I add banners to my threads?

[Mr Mendelli]

I've seen a few threads on Android Central with custom images on them. Some even use what looks like animated GIFs or perhaps videos that aren't within the thread. I have made a couple of tutorials with images, and the thread pulls the first image as the banner. I would like to set a custom banner that I choose. How is this done, and what are the dimensions for these banners?

 

[B. Diddy]

Moved to the Site Feedback forum.