function wfImagesInMD(ast, file, setting) {
  visit(ast, 'image', function(node) {
    if (node.url && node.url.trim().length > 0) {
      doesImageExist(file, node.url, node);
    }
  });
}
Remark Lint Test - verifies markdown images files exist.

@param {Object} ast
@param {Object} file
@param {Object} setting