There is. Internally, the spriteNodeWithImageNamed:
class method just uses the image name you pass it to set the node's texture property. That being said, if at any point you want to arbitrarily change the node's texture, you can just set it directly.
[self.sprite setTexture:[SKTexture textureWithImageNamed:@"someOtherImage"]];
There are also some SKActions for doing this, in case you want the node to resize or animate between different textures.
[self.sprite runAction:[SKAction setTexture:[SKTexture textureWithImageNamed:@"someOtherImage"] resize:YES]];
[self.sprite runAction:[SKAction animateWithTextures:@[tex1,tex2,tex3] timePerFrame:0.5 resize:YES restore:YES]];