This paper addresses knowledge-based authentication systems in self-service technology, presenting the design and evaluation of the Visual Identification Protocol (VIP). The basic idea behind it is to use pictures instead of numbers as a means for user authentication. Three different authentication systems based on images and visual memory were designed and compared with the traditional Personal Identification Number (PIN) approach in a longitudinal study involving 61 users. The experiment addressed performance criteria and subjective evaluation. The study and associated design exploration revealed important knowledge about users, their attitudes towards and behaviour with novel authentication approaches using images. VIP was found to provide a promising and easy-to-use alternative to the PIN. The visual code is easier to remember, preferred by users and potentially more secure than the numeric code. Results also provided guidelines to help designers make the best use of the natural power of visual memory in security solutions.