The more coarsely you filter the data (reducing resolution is essentially a low-pass filter on the image data), the more you reduce the bandwidth for a watermar

The more coarsely you filter the data (reducing resolution is essentially a low-pass filter on the image data), the more you reduce the bandwidth for a watermarking signal, but using spread spectrum and forward error correction techniques, the ratio of watermark to data can be brought arbitrarily low. There's no amount of obfuscation that will defeat watermarking if they algorithm/key is unknown and a huge amount of data needs to be released. That is, maybe you can use video and audio filtering/manipulation to push the watermark bandwidth down to one bit of watermark per 1 GB of data, but with 100 participants and 7 GB of data, 7 bits is enough to identify the leaker.