Screenshot'ing directly from a source that's as high quality as possible, whether it's from a Blu-Ray/DVD, Youtube clip, downloaded mp4, etc.. The higher quality, the better. 1080p>720p>480p>360p... Do you best to find the most HD source. It's unfortunate, because sometimes the best you can find is 360p.
Once the image is cropped down to 210px x 240px and <20KB, the image will be compressed anyway, but a compressed image from a 1080p source will look better than a compressed image from a 360p source.
