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.
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 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 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.