One way that a negative self-image can affect creativity is that makes individuals less likely to find creative motivation or listen to peers instead of generating her own ideas. Positive self-image helps an individual become more self-actualizing and independent, both of which aid creativity.
A negative self-image makes it less likely for a person to engage in any creative endeavor because she does not feel others appreciate what she has done. This is why it is important for parents to applaud the creative efforts of their children to promote the growth of that creativity. Similarly, a negative self-image may make an individual feel she is being judged for a creative endeavor that is inferior to someone else's, which is likely to reduce her creative output.
Positive self-image can affect creativity because it helps the individual feel she is respected and that the identity she expresses through creativity is valid and understood. Those with negative self-images are much likelier to allow the criticism of others to stifle their work.
Ultimately, self-image is tied to one's belief about herself. Those with negative self-images may harshly criticize themselves as severely as any external critic. However, positive self-image makes it likelier for her to acknowledge artistic triumphs rather than artistic failures and to continue producing creative works.