Introducing Image Check: Never Send an Email with Broken Or Slow-Loading Images Again

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We’re excited to launch a new feature in Litmus: Image Check. Never send an email with broken or slow-loading images again—Image Check identifies images that can cause problems and hurt click and conversion rates.

Testing emails before every send is crucial. Litmus tools make it easy to preview your email in webmail, desktop, and mobile inboxes, verify that links are working, optimize for image blocking, and improve accessibility. Adding Image Check to your email marketing toolkit ensures a seamless experience for users, better performing emails, and peace of mind before pressing send.

When images in email are broken or fail to load, subscribers miss out on your message or worse, your call-to-action. Conversions can also seriously suffer—modern web users are easily frustrated by images that take too long to load. In fact, a one-second delay in load time can lead to a 7% loss in conversions.


image check

When you start an email previews test in Litmus, Image Check is automatically run in the background while we process your email. Image Check will show you how many images are in your email, the size of each image, and the total size of all of the images included in your email. We’ll also show you the total time it takes for all the images in your email to load. In the detailed report, you can see the load time for each image, the full image path, and even click to open the image in a new tab. We’ll also identify any broken images with a red X.

How do images typically load?

The orange bars in each email’s Image Check report are staggered, appearing as a waterfall—and for many emails you’ll find that the images appear to load in groups of 4-5 at a time.

image loading

Typically, images for an email are all hosted on the same domain. There is a limit to the number of simultaneous connection requests that an email client can make to the same domain. Many people won’t be aware that email clients will only open between 4-6 images at once. For example, while Outlook and most desktop clients load four images simultaneously, iOS loads five.

In most cases this won’t affect the user experience in an obvious way—but if an email has a lot of “heavy” images, or is loading over a slow connection, you might find that images toward the bottom of an email will appear slowly, only after the images toward the top have loaded. This is a due to the “simultaneous connection limit.” As a result, when checking your Image Check results, it’s common to see the first 4-5 images grouped together in the chart. This chart showcases the order in which your images will load for your subscribers.


The answer is obvious here—the faster your images load the better! When it comes to web pages, users typically expect a site to load in two seconds or less and will start abandoning after three seconds. At a three-second load time, a 7% loss in conversions has already occurred. While these statistics may be web-focused, we anticipate them to be similar for email subscribers—who are equally as impatient.

Average image load time

We looked at 100,000 emails tested in Litmus and found the average time to download all images in an email is 2.19 seconds. However, nearly 12% of emails took longer than three seconds before all their images downloaded, and 7% of emails took longer than five seconds. At the ten second mark, a little over 3% of emails were still waiting for images to load—a huge conversion killer.


The file sizes and quantity of your images both play a factor in how long your images take to load. As expected, the smaller the file size and the lower the quantity of images, the faster the load time.

While there’s no specific guidelines or limits for image sizes, we recommend optimizing image sizes to be as small as possible without compromising quality. Out of the 100,000 emails that were part of our test, the average file size per image was 2.3kb.

Some other interesting facts we found:

  • Half of all images surveyed were under 3.7kb.
  • 75% of all images surveyed were under 15.6kb.
  • However, there were massive outliers. The largest image file was over 86mb (woah!).

Similarly, there isn’t a limit for the number of total images in your email. For numerous reasons—including images-off optimization and image load time—we recommend using a proper balance of live text and images to display your message. If you find that the images in your email are taking too long to display, consider using live text or background colors in place of some of your images.

Decreasing your image file size

If you don’t have the option of removing images in a campaign, your best bet is to reduce the file size of those images. In fact, you should always strive to reduce the file size of any images in your campaign. Fortunately, there are a number of tools available to help, without reducing the quality of the images themselves:

Retina images

While retina images are becoming increasingly popular and are great choice for users with high density screens (like the iPhone 6), they often contribute to large file sizes. There are several solutions to this issue, one of which involves scaling the image to 4x and then saving the image at 0 quality. This technique decreases file sizes, while having minimal impact on quality.

Animated GIFs

While animated GIFs are a great way to engage your subscribers, they can often have very large file sizes. Extremely large GIFs can cut into a subscriber’s data plan and be slow to load and play—both of which are frustrations that no audience should have to deal with. Fortunately, there are a number of methods for creating GIFs and reducing their file size, which include:

  • Cropping the image as much as possible.
  • Remove as many frames as you can without losing the illusion of motion.
  • Don’t animate the entire image—use layers in Photoshop to isolate animated parts and only animate those layers.
  • Reduce the number of colors saved in the file.


Ensure that you’re providing your subscribers with the best email experience possible—and aren’t missing out on conversions by testing before you send. Verify that your emails are displaying as intended, your links are going to the right landing page, and your images are loading properly (and quickly!) with Litmus—your all-in-one testing solution.

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