You will not have to get a second BA to get an MA in Psych. In fact, many graduate programs look favorably on someone coming in from another field who has a different perspective. You will have apply and get entrance to a program (this should be fairly easy if you have a good undergrad GPA and do well on the GRE). You will probably have to write an essay too and you should definitely play up the consumer psych link you mentioned and how your training in graphic design plays into that. You will have to complete the hours required (usually 30-35 semester hours) and any exams or thesis requirements. Every program is a little different in what they require to get the degree – but you certainly don’t have to go back and get another BA.

Good luck!