Portfolios have recently gained traction within computer science education as a way to assess students' computational thinking and practices. Whereas traditional assessments such as exams tend to capture learning within artificial settings at a single point in time, portfolios provide more authentic opportunities to document a trajectory of students' learning and practices in everyday contexts. Furthermore, because communication itself has been defined as an important computational thinking practice, portfolios give students a place to practice this skill in the classroom. In this study, we report on the implementation of a digital portfolio with a class of 21 high school students used to capture the process of creating of an electronic textile mural project. While students' understanding of computational concepts were only partially captured within the portfolios, their engagements with computational practices -- such as debugging and iteration -- were better highlighted. Much of this was due to the students' existing communicative strategies themselves, both in terms of how precise they were in describing issues, as well as how they leveraged images and code to explain their process. Recommendations for designing more effective portfolio assessments are discussed, which include greater emphasis on creating shared classroom discourse, and leveraging students' existing experiences with multimedia.