Advice on image resolutions and retina displays


PNG and GIF images
JPG images
   » JPG quality loss
Getting the best for your retina display


The platform tries to make sure images are of as high a quality as they can be, without causing download sizes that are too large for recipients. If download sizes are large, this could affect a recipient's mobile data usage and the speed at which the image is displayed.

If you're unsure what image sizes to use, or don't want to have to resize your own images, then we recommend uploading JPG images. We'll then do the rest.

However, if you want more control over your images, then here's a quick guide to what we do with different formats.

PNG and GIF images

We don't adjust PNG or GIF images. If an image is uploaded at 1000px wide, it will be sent at that same resolution.

If you wish to scale your PNG of GIF images, you can use the picture editor available from within the image manager.

Read more about editing images in the image manager »

JPG images

JPG images are handled differently. We re-scale these, depending on where and how they're used. If you place a high-resolution JPG image (e.g. 1000px wide) into a campaign at 300px wide on-screen, we'll re-scale the original image to 300px wide to match this.

This helps keep file sizes down.

However, in all of our mobile responsive campaigns (which most are), we actually have a minimum JPG image width of 640px. This ensures the image remains crisp on high-DPI devices, such as phones and retina screens.

For the record, modern iPhones are 750px wide, whereas the older ones are 640px wide. Generally, on either type of device, 640px wide images tend to look quite sharp.

JPG quality loss

JPG is a lossy format, meaning they're compressed in such a way that reduces the quality of the original source image. When we re-scale, we use a quality value of 90%, meaning there will be a slight drop in quality. For the most part, this won't be noticeable, but occasionally you may notice that the final result isn't as good as the original. In these cases, we'd recommend using a PNG or GIF instead.

Getting the best for your retina display

Generally, whatever format you use, you should make sure your images are at least 600px wide when you upload them to the platform.

JPG images tend to be smaller in terms of file size than their PNG counterparts - especially for photography. Therefore, for the most part, we'd recommend using JPG files.

For logos or icons, however, PNG or GIF tend to be a better option in terms of quality, and sometimes file size.

If you're trying to get a full-width image to work well on a high-DPI desktop display (such as a Mac with a retina screen), then you're best off uploading such images as PNG files at around 1200px (or twice the width of your campaign). JPG files, in this instance, will be downscaled to the width of the campaign.

If image sharpness is of important to you, then we recommend using PNG (or PNG-24) for your images. These will ensure that your images remain as sharp as the originals on all devices. But do be mindful of file sizes! Many people have limited bandwidth on mobile contracts, so forcing them to download a large number of extremely high-resolution images will have an impact on that.

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    Hi Rikki. Thanks for the article.

    So is the 100kb recommended limit warning in Dotmailer redundant now with high res screens and faster download speeds? Especially if dotmailer automatically compresses JPGs?


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    Hi Jason - thanks for your comment. Essentially, yes, that recommendation is too small these days for the reasons you outline, and we're looking at bringing it up to date (it was initially written a while ago now when it was more relevant!).

    There are of course still some sensible guidelines that are worth bearing in mind for images in emails, such as how lower sizes can minimise the amount of time it takes for images to download over mobile networks (especially in circumstances when connectivity isn’t very good), and being wary of how a low text-to-image ratio can possibly trigger spam filters. However, we appreciate your raising this and we're in the process of reviewing our recommendation.

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    Hi Neal. Has there been any update to this since October?

    It would be very useful to have this as an account or image management option rather than having it at a fixed level of compression. Typically for our email campaigns we have an offer or discount within our hero image, which we would usually save as a jpg (already compressed at 70-80% in Photoshop). I'm well aware of best practices for email marketing however, this feels like it's currently setup the wrong way around. File sizes are always something to consider, but in its current state the quality of the offer within the hero image is being compressed too heavily causing it to look blurry (especially with our lighter font weights), meaning we don't really have a choice but to use PNG images which are about 1MB more in file size.

    Edited by Graham Hart
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    @Graham - after reviewing the advice given on the summary page, we'll be removing the image size warning in the near future. We'll be keeping the overall HTML size warning, as both Gmail and Yahoo have limits here that are worth adhering to for the most part, but the help text will make it clearer why it matters.

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    @Michael - Thanks for your response. My point wasn't really about an image size warning, it was more to do with the system resizing / compressing JPG files that are already optimised, causing them to distort. We also use TinyPNG and TinyJPG for images, to compress them to an appropriate size (while retaining the image quality and dimensions for retina displays etc).

    Ideally we would just want to switch off image auto resizing and compression for JPG files. Is this possible? When I go into image manager it has the correct dimensions, but when I add it to a campaign, it resizes it making it look poor quality.

    I noticed when I preview a campaign, there are parameters being added to the image URL (email80_living.jpg?w=660&h=0&cid=) which seems to resize and compress the image. When I remove these parameters from the end of the file name, it uses the original image. This only affects JPG files but I guess this is the part we'd like to switch off!

    Around half of our recipients open our emails with an iPad, so we really want the images to display as best as possible on their retina displays, especially where there is text on an image.

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    @Graham You shouldn't need to compress or resize any JPEG images before uploading them (unless they're over 10MB), as we'll take care of the resizing at point of send.

    We talk about Retina/high DP screens in the article itself (, but I acknowledge this means using PNG files; we may need to think about how we can get better at this, so thanks for the feedback.