Systems and methods for pixel-based watermarking are disclosed. An online advertising platform, at a first time, receives information identifying a creative that is associated with an advertiser or publisher, and associates a unique identifier with the creative. The color of one or more pixels in the creative image are modified based on the unique identifier by, for example, converting the unique identifier to a corresponding color value and setting the pixels to the color. At a second, later time, the platform receives a second image comprising at least a portion of the creative image. One or more pixels are identified in the second image that correspond to pixels in the first image that were modified in color, and the unique identifier is derived based on the color of the identified pixels in the second image. Information associated with the advertiser or publisher is then retrieved based on the determined identifier.