In some software company, who should be responsible for the UI design:
User
Designer
Manager
Boss
Depends on company size
etc.
In UI design I mean not only colors and images, but also control's layout, count, size, style, may be text user see.
In a small company, the answer is "whoever is good at it". Some of our best graphics were designed by a technical author who happened to have a flair for graphic design. Don't assume that someone has to have the right job title to do a creative job - innate talent trumps a job title any day!