Well, found another missing node in Poser. It's the Scale node.

It is what's known as a distort node. Here's what the manual says:

For instance X: 0.5 and Y: 0.5 would produce an image which width and height would be half of what they used to be.

Use this node to match image sizes. Most nodes produce an image that is the same size as the image input into their top image socket. To uniformly combine two images of different size, the second image has to be scaled up to match the resolution of the first.

Boni suggests using the Firefly 2D Texture Scale node instead.