Hi Tom,
You can hire Marketo's team to do this for you (they charge though) or you can hire an html and web designer. Give them the code from a New blank tempalte and they should be able to code it from there.
Be sure they use absolute IMG URLs because you'll need to upload the images to Marketo first, take those URLs and insert into the templates.
Email Templates are easier - just have the designer create a responsive (or not) template that follows good email design practices. There are some good resources if you look up "best practice email design" or check out templates.marketo.com.