By default, the child variation images will be pulled from the description image set in each variation's image manager. That image will be sent to the channel to display when selecting the variation.
Manually Set Images
However, you manually set any image to be used. Open the toolbox on the Matrix parent, press Matrix, then press Image Manager on the sub-menu. Next to each variation, you can either select any image already loaded on the variation, or select an image from a local file on your PC.
Bulk Update Images
You can also update variation images in bulk with a file upload. Navigate Inventory > Import Product Info > Import Product N-matrix Images. Enter the image URL next to the defined matrix parent and dimension, and upload.