Facebook covers: 851x315 pixels, JPG, and under 100KB if you want Facebook to leave it alone. Drop an image and it comes back at exactly that spec, cropped where you choose.
The two-crops problem: you upload one image and Facebook shows two different crops of it. Desktop displays 820x312, nearly the full frame. Phones take the centre 560 pixels and show them taller, and almost 150 pixels vanish from each side. The preview above shades what phones lose. Keep faces and text out of the shaded strips and the cover works everywhere.
Why the 100KB cap: Facebook recompresses covers, and its own guidance says an sRGB JPG under 100KB skips the worst of it. The tool searches for the best quality that fits. What you upload is what people see rather than what a compressor made of it.
Fitting the shape: 851x315 is a wide letterbox. Landscapes and abstract textures take it well, while group shots lose heads. The crop slider picks which band of the image is kept, and a few options can go through together so they can be compared at the size Facebook will show them.