diff --git a/core/server/helpers/img_url.js b/core/server/helpers/img_url.js
index a41e4f17db..2527fde7fc 100644
--- a/core/server/helpers/img_url.js
+++ b/core/server/helpers/img_url.js
@@ -8,61 +8,96 @@
 // `absolute` flag outputs absolute URL, else URL is relative.
 
 const url = require('url');
+const _ = require('lodash');
 const proxy = require('./proxy');
 const urlService = proxy.urlService;
 const STATIC_IMAGE_URL_PREFIX = `/${urlService.utils.STATIC_IMAGE_URL_PREFIX}`;
 
-module.exports = function imgUrl(attr, options) {
-    // CASE: if no attribute is passed, e.g. `{{img_url}}` we show a warning
+module.exports = function imgUrl(requestedImageUrl, options) {
+    // CASE: if no url is passed, e.g. `{{img_url}}` we show a warning
     if (arguments.length < 2) {
         proxy.logging.warn(proxy.i18n.t('warnings.helpers.img_url.attrIsRequired'));
         return;
     }
 
-    const absolute = options && options.hash && options.hash.absolute && options.hash.absolute !== 'false';
-
-    const size = options && options.hash && options.hash.size;
-    const imageSizes = options && options.data && options.data.config && options.data.config.image_sizes;
-
-    const image = getImageWithSize(attr, size, imageSizes);
-
-    // CASE: if attribute is passed, but it is undefined, then the attribute was
+    // CASE: if url is passed, but it is undefined, then the attribute was
     // an unknown value, e.g. {{img_url feature_img}} and we also show a warning
-    if (image === undefined) {
+    if (requestedImageUrl === undefined) {
         proxy.logging.warn(proxy.i18n.t('warnings.helpers.img_url.attrIsRequired'));
         return;
     }
 
-    if (image) {
-        return urlService.utils.urlFor('image', {image}, absolute);
+    // CASE: if you pass e.g. cover_image, but it is not set, then requestedImageUrl is null!
+    // in this case we don't show a warning
+    if (requestedImageUrl === null) {
+        return;
     }
 
-    // CASE: if you pass e.g. cover_image, but it is not set, then attr is null!
-    // in this case we don't show a warning
+    // CASE: if you pass an external image, there is nothing we want to do to it!
+    const isInternalImage = detectInternalImage(requestedImageUrl);
+    if (!isInternalImage) {
+        return requestedImageUrl;
+    }
+
+    const {requestedSize, imageSizes} = getImageSizeOptions(options);
+    const absoluteUrlRequested = getAbsoluteOption(options);
+
+    function applyImageSizes(image) {
+        return getImageWithSize(image, requestedSize, imageSizes);
+    }
+
+    function getImageUrl(image) {
+        return urlService.utils.urlFor('image', {image}, absoluteUrlRequested);
+    }
+
+    function ensureRelativePath(image) {
+        return urlService.utils.absoluteToRelative(image);
+    }
+
+    // CASE: only make paths relative if we didn't get a request for an absolute url
+    const maybeEnsureRelativePath = !absoluteUrlRequested ? ensureRelativePath : _.identity;
+
+    return maybeEnsureRelativePath(
+        getImageUrl(
+            applyImageSizes(requestedImageUrl)
+        )
+    );
 };
 
+function getAbsoluteOption(options) {
+    const absoluteOption = options && options.hash && options.hash.absolute;
+
+    return absoluteOption ? !!absoluteOption && absoluteOption !== 'false' : false;
+}
+
+function getImageSizeOptions(options) {
+    const requestedSize = options && options.hash && options.hash.size;
+    const imageSizes = options && options.data && options.data.config && options.data.config.image_sizes;
+
+    return {
+        requestedSize,
+        imageSizes
+    };
+}
+
+function detectInternalImage(requestedImageUrl) {
+    const siteUrl = urlService.utils.getBlogUrl();
+    const isAbsoluteImage = /https?:\/\//.test(requestedImageUrl);
+    const isAbsoluteInternalImage = isAbsoluteImage && requestedImageUrl.startsWith(siteUrl);
+
+    // CASE: imagePath is a "protocol relative" url e.g. "//www.gravatar.com/ava..."
+    //       by resolving the the imagePath relative to the blog url, we can then
+    //       detect if the imagePath is external, or internal.
+    const isRelativeInternalImage = !isAbsoluteImage && url.resolve(siteUrl, requestedImageUrl).startsWith(siteUrl);
+
+    return isAbsoluteInternalImage || isRelativeInternalImage;
+}
+
 function getImageWithSize(imagePath, requestedSize, imageSizes) {
-    if (!imagePath) {
-        return imagePath;
-    }
     if (!requestedSize) {
         return imagePath;
     }
 
-    const blogUrl = urlService.utils.getBlogUrl();
-
-    if (/https?:\/\//.test(imagePath) && !imagePath.startsWith(blogUrl)) {
-        return imagePath;
-    } else {
-        // CASE: imagePath is a "protocol relative" url e.g. "//www.gravatar.com/ava..."
-        //       by resolving the the imagePath relative to the blog url, we can then
-        //       detect if the imagePath is external, or internal.
-        const resolvedUrl = url.resolve(blogUrl, imagePath);
-        if (!resolvedUrl.startsWith(blogUrl)) {
-            return imagePath;
-        }
-    }
-
     if (!imageSizes || !imageSizes[requestedSize]) {
         return imagePath;
     }
diff --git a/core/test/unit/helpers/img_url_spec.js b/core/test/unit/helpers/img_url_spec.js
index 2662b83bac..aa7cfec88a 100644
--- a/core/test/unit/helpers/img_url_spec.js
+++ b/core/test/unit/helpers/img_url_spec.js
@@ -32,6 +32,13 @@ describe('{{image}} helper', function () {
         logWarnStub.called.should.be.false();
     });
 
+    it('should output relative url of image if the input is absolute', function () {
+        var rendered = helpers.img_url('http://localhost:82832/content/images/image-relative-url.png', {});
+        should.exist(rendered);
+        rendered.should.equal('/content/images/image-relative-url.png');
+        logWarnStub.called.should.be.false();
+    });
+
     it('should output absolute url of image if the option is present ', function () {
         var rendered = helpers.img_url('/content/images/image-relative-url.png', {hash: {absolute: 'true'}});
         should.exist(rendered);
@@ -120,7 +127,7 @@ describe('{{image}} helper', function () {
                 }
             });
             should.exist(rendered);
-            rendered.should.equal('http://localhost:82832/content/images/size/w400/my-coole-img.jpg');
+            rendered.should.equal('/content/images/size/w400/my-coole-img.jpg');
         });
         it('should output the correct url for protocol relative urls', function () {
             var rendered = helpers.img_url('//website.com/whatever/my-coole-img.jpg', {