Set image to thread title

[Mr Mendelli]

I've seen some threads on Android Central with custom images on them. Some even use what looks like animated GIFs or 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? Here's an example, the image displayed was found nowhere within the thread its self:

Here's the thread in the image just in case I'm the one not seeing the it...