The correct answer is this
Operation: HTTP DELETE
URL : www.shop.com/api/images/products/<product-id>/<image-id>
If you are using the webservice API then
resource = "images/products/<product-id>"
id = <image-id>
i.e.
$webService->delete(array('resource' => 'images/products/<product-id>', 'id' => '<image-id>'));