Actually, there are people in this field who only knows the designing part of it and there are people (like me

) who only know the programming part of it. And there are people who know both.
But usually, one type of people take help of another type of people and get the work done.

For example, I always take help of a professional visualiser to give custom templates to clients. So I really don't feel any need to urge photoshop/gimp or upgrade my skills in that area.