Instructing an artist to create a painting is known as "commissioning" a painting. It is something that requires some thought because the end result will, to a large extent, depend on how good the communication is between the artist, and the person commissioning the painting. The artist can feel very restricted and the purchaser can feel he or she has no control over what they are paying for.
The extent to which the artist is left to do their own thing, allows them more room to be creative, and the results can be good, or bad depending on the requirements of the purchaser. So many artists shy away from taking on commissions for fear of disappointing the purchaser.