diff --git a/inc/class-img-shortcode.php b/inc/class-img-shortcode.php
index 25f57ea..36bf499 100644
--- a/inc/class-img-shortcode.php
+++ b/inc/class-img-shortcode.php
@@ -176,37 +176,44 @@ public static function callback( $attr, $_null, $shortcode_tag ) {
 		 * @param array Shortcode attributes, decoded and merged with defaults.
 		 */
 		$attr = apply_filters( 'img_shortcode_attrs', $attr );
-
-		$image_html = '<img ';
-
 		$image_classes = explode( ' ', $attr['classes'] );
 		$image_classes[] = 'size-' . $attr['size'];
 		$image_classes[] = $attr['align'];
+		$image_classes_string = trim( implode( ' ', $image_classes ) );
+
+		if ( isset( $attr['attachment'] ) ) {
+			$attrs_for_removal = array(
+				'caption' => null,
+				'attachment' => null,
+				'linkto' => null,
+				'url' => null,
+				'size' => null,
+				'align' => null,
+				'src' => null,
+				'classes' => null,
+			);
+			$attr_for_responsive = array_diff_key( $attr, $attrs_for_removal );
+			$attr_for_responsive['class'] = $image_classes_string;
+			$image_html = wp_get_attachment_image( $attr['attachment'], $attr['size'], false, $attr_for_responsive );
+		} else if ( ! empty( $attr['src'] ) ) {
+			$image_html = '<img ';
 
-		$image_attr = array(
-			'alt' => $attr['alt'],
-			'class' => trim( implode( ' ', $image_classes ) ),
-		);
+			$image_attr = array(
+				'alt'   => $attr['alt'],
+				'class' => $image_classes_string,
+				'src'   => esc_url( $attr['src'] ),
+			);
 
-		if ( isset( $attr['attachment'] ) &&
-				$attachment = wp_get_attachment_image_src( (int) $attr['attachment'], $attr['size'] ) ) {
-			$image_attr['src'] = esc_url( $attachment[0] );
-			$image_attr['width'] = intval( $attachment[1] );
-			$image_attr['height'] = intval( $attachment[2] );
-		} else if ( ! empty( $attr['src'] ) ) {
-			$image_attr['src'] = esc_url( $attr['src'] );
+			foreach ( $image_attr as $attr_name => $attr_value ) {
+				if ( ! empty( $attr_value ) ) {
+					$image_html .= sanitize_key( $attr_name ) . '="' . esc_attr( $attr_value ) . '" ';
+				}
+			}
+
+			$image_html .= '/>';
 		} else {
 			return; // An image without a src isn't much of an image
 		}
-
-		foreach ( $image_attr as $attr_name => $attr_value ) {
-			if ( ! empty( $attr_value ) ) {
-				$image_html .= sanitize_key( $attr_name ) . '="' . esc_attr( $attr_value ) . '" ';
-			}
-		}
-
-		$image_html .= '/>';
-
 		/**
 		 * Filter the output of the <img> tag before wrapping it in link or caption
 		 *
@@ -234,8 +241,10 @@ public static function callback( $attr, $_null, $shortcode_tag ) {
 		if ( ! empty( $attr['caption'] ) ) {
 
 			// The WP caption element requires a width defined
-			if ( empty( $attr['width'] ) ) {
-				$attr['width'] = $image_attr['width'];
+			if ( empty( $attr['width'] ) && isset( $attr['attachment'] ) &&
+				$attachment = wp_get_attachment_image_src( (int) $attr['attachment'], $attr['size'] )
+				) {
+					$attr['width'] = intval( $attachment[1] );
 			}
 
 			$image_html = self::captionify( $image_html, $attr );
diff --git a/tests/test-img-shortcode.php b/tests/test-img-shortcode.php
index 70b8d95..8cb3589 100644
--- a/tests/test-img-shortcode.php
+++ b/tests/test-img-shortcode.php
@@ -9,6 +9,7 @@ class Test_Img_Shortcode extends WP_UnitTestCase {
 	// @codingStandardsIgnoreStart
 	public function setUp() {
 		parent::setUp();
+		switch_theme('twentyfifteen');
 
 		$this->attachment_id = $this->insert_attachment( null,
 			dirname( __FILE__ ) . '/data/fusion_image_placeholder_16x9_h2000.png',
@@ -22,7 +23,6 @@ public function setUp() {
 		);
 
 		$upload_dir = wp_upload_dir();
-
 		$this->image_src = $upload_dir['url'] . '/fusion_image_placeholder_16x9_h2000.png';
 		$this->image_path = $upload_dir['path'] . '/fusion_image_placeholder_16x9_h2000.png';
 	}
@@ -38,6 +38,10 @@ function test_construct_ui() {
 		// replace this with some actual testing code
 		$this->assertTrue( true );
 	}
+	function test_theme() {
+		// replace this with some actual testing code
+		$this->assertContains( 'Twenty Fifteen', wp_get_theme()->name );
+	}
 
 
 	/*
@@ -79,6 +83,11 @@ function test_img_shortcode_from_attachment() {
 		$expected_href_attr = get_permalink( $attachment_id );
 		$this->assertContains( 'href="' . $expected_href_attr . '"', $content );
 
+		// Test srcset
+		$content = apply_filters( 'the_content', '[img attachment="' . $attachment_id . '" linkto="attachment" /]' );
+		$srcset = 'srcset="http://example.org/wp-content/uploads/2015/12/fusion_image_placeholder_16x9_h2000-300x169.png 300w, http://example.org/wp-content/uploads/2015/12/fusion_image_placeholder_16x9_h2000-768x432.png 768w, http://example.org/wp-content/uploads/2015/12/fusion_image_placeholder_16x9_h2000-1024x576.png 1024w" sizes="(max-width: 2000px) 100vw, 2000px"';
+		$this->assertContains( $srcset, $content );
+
 		// Test caption attribute
 		$caption = <<<EOL
 This is a "<em>caption</em>". It should contain <abbr>HTML</abbr> and <span class="icon">markup</span>.