1. A computer-implemented method comprising: receiving, at a first time, information identifying a creative comprising a first image, the creative being associated with an advertiser or a publisher; associating a unique identifier with the creative; selecting a pixel at a location in the first image; modifying a color of the pixel at the location in the first image based on the unique identifier; receiving, at a second time later than the first time, a second image comprising at least a portion of the creative image; identifying a pixel at a location in the second image corresponding to the pixel at the location in the first image; determining the unique identifier based on a color of the pixel at the location in the second image; retrieving information associated with the advertiser or the publisher based on the determined unique identifier; holding an auction for serving the creative to an impression consumer; associating a second unique identifier with the auction; selecting a pixel at a second location in the first image; modifying a color of the pixel at the second location in the first image based on the second unique identifier; identifying a pixel at a second location in the second image corresponding to the pixel at the second location in the first image; determining the second unique identifier based on a color of the pixel at the second location in the second image; and retrieving information associated with the auction based on the determined second unique identifier.