Actually, there is a way to know if the image will result in the Twitch logo. When you download the game logo, if it doesn’t exists, you will be redirected to another image.
For instance, if you search for the game Pokémon FireRed/LeafGreen and try to get the logo 240x144 (http://static-cdn.jtvnw.net/ttv-logoart/Pok%C3%A9mon%20FireRed%2FLeafGreen-240x144.jpg), the result is a Twitch logo because it’s missing… but look at the URL, you are being redirected to http://static-cdn.jtvnw.net/ttv-static/404_preview-240x144.jpg
For every missing logo, you will be redirected to http://static-cdn.jtvnw.net/ttv-static/404_preview-{width}x{height}.jpg
Thanks,
Schmoopiie!