More than the tools he uses a Graphic / UI Designer must be creative. Of-course without the knowledge of the tools creativity cannot be published, so we expect our designers to have a grip on one or more of the popular raster editing software like Gimp, PhotoShop, Krita, PaintShop etc. He / She must have the knowledge of any of the popular vector graphics tools like InkScape, Corel Draw, Illustrator etc. He / She must have a good understanding of UI elements and specifically UI elements pertaining to the web interfaces.