Although the emphases on the importance of proving in mathematics education literature, many studies show that undergraduates have difficulty in this regard. Having researchers discussed these difficulties in detail; many frameworks have been presented evaluating the proof from different perspectives. Being one of them the proof image, which takes into account both cognitive and affective factors in proving, was presented by Kidron and Dreyfus (2014) in the context of the theoretical framework of "abstraction in context". However, since the authors have not deepened the relationship between the proof image and formal knowledge, this article was intended to fill this gap. In this study, which is part of a larger doctoral thesis, descriptive method one of the qualitative methods was used. The participants of the study were three pre-service teachers selected via criterion sampling method among sophomore elementary school mathematics teacher candidates. In parallel with a course relating to Cantorian Set Theory, task-based individual interviews (Task I-II-III-IV) were conducted in the context of the equivalence of infinite sets. The subject of "infinity" had been chosen as the context of the study since it contains a process that goes from intuitive to formal. In the first task (Task I), the actions that the participants had performed without enough pre-knowledge was examined in terms of the proof image. In the second task (Task II) carried out after a course, in which basic knowledge was presented, the same question was asked to the participants again. Thus, the processes formed with the presence of formal knowledge were analysed. As a result of the descriptive analysis executed on the data of both tasks, it was determined that Ç, who was one of the participants, reached a proof image in the second task although she failed in the first task. Therefore, in this study, findings of her proving activity were shared. Consequently, formal knowledge has been identified to be directly related to each of the components of the proof image and, its main contributions have been listed as headings.