There's probably compact signatures extracted from the screenshots (color profiles, OCR, etc) which are then uploaded later in bulk. You don't need the full original image to be able to reliably uniquely identify the content if you have an index of it already.

I'm wondering if there is some sort of steganographic watermark that broadcasters are including in media, to enable stuff like this. Probably would need to be robust in the presence of re-encoding, more compression, etc..