Well, using modern Swift (you're using Swift by now, right?), you could try:
var marbleNode: SKSpriteNode!
Then later, in your init method:
marbleNode = SKSpriteNode(imageNamed: "SmallerSwirl");
marbleNode.physicsBody = SKPhysicsBody(circleOfRadius: 35.0)
marbleNode.physicsBody?.dynamic = true
marbleNode.physicsBody?.affectedByGravity = true
print(marbleNode.physicsBody)
marbleNode.position = CGPointMake(centerPoint.position.x + 10.0, centerPoint.position.y + 10.0)
self.addChild(marbleNode)
Okay, so that gives us a round sprite node to work with. The Sprite node is responsive to physics, because you set up its physics body separately. So far, so good. Now we need to address the glossed-over part, namely the introduction of the SmallerSwirl .png image.
When you set up your project, it included an Assets.xcassets (pronounced, "x c assets") entry. Click on it, then click on the "+" sign at the middle/bottom of the first column, by the word filter. From the menu that appears, select "New Image Set". A new entry labeled "Image" appears. Click on the word "Image" to change it to "SmallerSwirl".
Next to the SmallerSwirl entry, you see blanks labeled 1x, 2x, and 3x. They are for different screen resolutions. Start by dragging your preferred .png image into the 1x square. That image can be named whatever you want it to be named. It doesn't have to be named SmallerSwirl, though it can be. Drag other images to the 2x and 3x slots if you like.
Run, and you should see your preferred image embodied as a sprite, dancing around the screen.