You should include a reference every time you quote, paraphrase (i.e. write someone's information in your own words), summarise or gain ideas from another person's work.
You should include a reference no matter what format the work takes (e.g. web page, journal article, email or diagram).
Many Departments and Faculties at the University of Strathclyde also produce their own specific guidance for citation requirements
It is therefore advisable that you confirm these with your department in the first instance.