Facebook cover photo should be 820x312 pixels for desktop display. It will appear as 640x360 on mobile. Facebook profile photos should be at least 180x180 pixels.
YouTube thumbnails should be 1280x720 pixels (16:9 ratio) in JPG, GIF, BMP, or PNG format under 2MB. This is the recommended size for best quality across all devices.
Resizing to smaller dimensions generally maintains good quality. Scaling up (making images larger) can reduce quality as pixels are interpolated. Always start with the highest resolution source image possible.