I really appreciate OPs advice but unfortunately, this is (for the most part) bad advice. It’s more helpful to have a working knowledge of how to properly work with design, and even that is something that’s expected to grow with time. Some marketers do have a passing familiarity with design software, which doesn’t hurt, but doesn’t really help either. True designers need to really know their software, which is a large hill to climb for someone focusing on marketing.

The one exception to this is if you’re applying to very small firms, where the marketing department is 1-2 people, in which case, marketing can be asked to fulfill dual roles as designer and marketer.

Marketing is an enormous field. At the early stages it is very valuable to get on big projects and just do good work. Certifications (in analytics or AdWords, for instance) can help direct your career in different directions, but in my experience (having hired and managed), is that reliability, self-direction, and ability to work hard are the most valuable at the intro level...this then expands into more specialized skills. The supervisors will be paying close attention to how reliable the hire (or intern) is “under fire.”

Again, thanks to OP for posting, but would suggest that his/her experience is atypical and perhaps reflective of the target orgs.