Masking is quite easy with Sprite Kit. Easier than I thought it would be, and you copy and paste this into the init statement of your Sprite Kit scene, you’ll see the always-included spaceship image get masked down to a 100 by 100 square.
SKSpriteNode *pictureToMask = [SKSpriteNode spriteNodeWithImageNamed:@"Spaceship"];
SKSpriteNode *mask = [SKSpriteNode spriteNodeWithColor:[SKColor blackColor] size: CGSizeMake(100, 100)]; //100 by 100 is the size of the mask
SKCropNode *cropNode = [SKCropNode node];
[cropNode addChild: pictureToMask];
[cropNode setMaskNode: mask];
[self addChild: cropNode];
cropNode.position = CGPointMake( CGRectGetMidX (self.frame), CGRectGetMidY (self.frame));
So we’ve got three objects here. The pictureToMask, mask, and cropNode. The pictureToMask is added as a child of the cropNode. The mask object isn’t actually added as a child of anything (oddly enough), but as you can see setMaskNode is called on the cropNode with the mask as a parameter. So the cropNode is basically a container with the pictureToMask in it, and the mask as a property.