The widespread use of digital image-processing software to prepare images for publication is a matter of growing unease among journal editors, particularly in the biosciences. Concerned not so much with intentional fraud, but rather with routine and 'innocent' yet inappropriate alteration of digital images, several high-profile science journals have recently introduced guidelines for authors regarding image manipulation, and are implementing in-house forensic procedures for screening submitted images. Such interventions can be seen as an attempt to 'draw a line' for the scientific community regarding acceptable and unacceptable practices in image production. However, in trying to define simple best-practice guidelines for digital image processing, these journals raise – perhaps inadvertently – a number of longstanding ambiguities concerning the role of images in the production and communication of scientific knowledge. This paper draws on recent image-processing guidelines and journal commentaries to analyse four key tensions relating to the production, circulation and interpretation of digital images in scientific publications. By examining where and how journal editors are drawing lines with respect to image-making practices, this case study explores how trust, the distribution of authority and accountability, and the nature of objectivity are being (re-) negotiated in the digital age.