In the product csv, you add all images. The will be loaded starting at position 1, 2, 3, etc. In the combinations csv you use the image position field, to link to the position of the image.
In my example all the red combinations reference image position 1 and 2, which in this example are the images for the red product. If only one image, reference just 1 image.
In the admin part with the product, the image top left is the first position, the next to it the second position, the one bottom right Count(position).
And if you want to add a new combiation (1 color 3 sizes) to an existing product. Count how many images it has. Put in the csv field "image URL" with the first combination a link(s) to the image(s). Leave "image position" empty for the first row. Leave "image URL" empty for the other 2 sizes and fill "image position" with (count + 1), (count + 2) depending on the amount of images you added with the first combination.
rg,
Leo