1. A system including: i) a user portable device comprising a display and a processor, the processor configured for steganographically-encoding plural-bit auxiliary data within artwork, and controlling display of encoded artwork on the display, the artwork indicating a payment service and including a VISA®, MasterCard® or American Express® logo, the steganographically-encoding concealing presence of the plural-bit auxiliary data from human viewers, the steganographically-encoding changing with time, requiring capture of plural frames over time to gather imagery sufficient to completely decode the plural-bit auxiliary data; and ii) a checkout system comprising a camera system, a processor, and one or more memories, in which buses provide an interconnection between the camera system and the one or memories, the one or more memories containing instructions configuring the checkout system to perform acts including: using the camera system to capture first image data depicting a product presented for purchase by a user; using the camera system to capture plural frames of second image data depicting the encoded artwork from the display of the user portable device; decoding the steganographically-encoded plural-bit auxiliary data from the captured plural frames of second image data, in which the decoding utilizes the processor, and in which the buses provide an interconnection between the one or more memories and the processor; deriving context information about the user portable device by processing the second image data captured by the camera system, the deriving yielding derived context information, in which the derived context information comprises information about pose or motion of the user portable device; and using the decoded plural-bit auxiliary data and the derived context information in connection with authenticating a payment transaction serviced by VISA®, MasterCard® or American Express®.