I want to use this image as a material for a SCKSphere:
I can do this as follows:
SCNSphere *sphereGeometry = [SCNSphere sphereWithRadius:5];
sphereGeometry.firstMaterial.diffuse.contents = [UIImage imageNamed:@"navball.png"];
I solved the problem by increasing the sphere geometry's
segmentCount value, which increases the detail of the polygon mesh used to render the shape, making it appear "smoother".
For more details see the SCNSphere documentation.