Sending one image to a million people is easy. Sending a million different images, each drawn for one subscriber at the moment they open the email, sounds like it should be much harder. It is, under the hood. But for marketers using a platform built for it, subscriber-specific images are now no more work than a regular image.
Here's how dynamic email images actually work at scale: what happens on each open, how caching and email proxies affect it, and what that means for designing and sending them.
You never create a million images. You create one design with variables in it: a name, a balance, a code, a product. That design lives on the rendering service. Your email contains an image address with each subscriber's values in it, filled in by your email platform's merge tags at send time.
The subscriber-specific image doesn't exist until someone opens the email. Then it's drawn on demand.
Multiply that by every open of every email, all happening at unpredictable moments, and the scale challenge becomes clear. Opens spike right after a send, at lunchtime, and whenever a big campaign lands.
People scroll fast. If an image takes too long to arrive, it loads after the reader has moved past, or shows as a blank space. A rendering service needs to draw images quickly and deliver them from servers close to the reader. That's why serious platforms deliver through a content delivery network (CDN), a network of servers around the world, rather than a single server.
Caching means keeping a copy of something so it can be served again without redoing the work. It's how the web stays fast. For subscriber-specific images, it's a balancing act.
Some things can be cached safely. Fonts, background artwork, and the fixed parts of a design don't change, so there's no reason to reload them each time.
Other things can't be cached at all. A countdown must show the time remaining at each open. A stock level or delivery status must reflect the latest data. So the service tells browsers and proxies not to keep old copies of those images.
Getting this balance wrong in one direction makes images slow. In the other, it makes them stale.
This is the part that surprises people. Gmail doesn't fetch images directly from the rendering service. It requests them through Google's own servers. Apple Mail, with Mail Privacy Protection on, routes image requests through Apple's servers too, and may load images when the email arrives rather than when it's read.
Two consequences follow. First, anything worked out from the request itself, especially the reader's location from their IP address, may reflect the proxy rather than the person. Second, proxies may try to reuse images, which is why rendering services send clear instructions not to cache time-sensitive ones.
For personalization based on your own data, the proxies don't matter. The values are in the image address, so the right image is drawn no matter who fetches it. That's why the most reliable approach is to pass the data you already have, like a stored city, rather than relying on detection at open.
Some emails use several subscriber-specific images at once: a reward amount in the header and its code further down, for example. They need to agree. Alterable can resolve the same recipient across every image in a send, so values stay consistent throughout the email.
Some content needs to recognize a subscriber across opens, like an evergreen countdown that remembers when someone first opened. That's done with a unique ID from your email platform, passed as a parameter. Using a generated ID instead of an email address keeps personal data out of image URLs.
You don't need to build or manage any of this. With Alterable, you design the image in a browser, set variables and fallbacks, copy an image tag, and paste it into your email platform. Rendering, CDN delivery, cache control, and proxy handling are taken care of.
What you should still own is the data: make sure the fields you use are clean, consistently formatted, and synced to your email platform. And test with real contacts, including ones with missing data, long names, and accented characters.
Because images are drawn on demand, you only pay for what's actually seen. Alterable counts an impression when an email containing its content is opened and the content fully loads. Emails that are never opened cost nothing. The free plan includes 10,000 impressions a month with no watermark, Pro includes 500,000 for $49 a month, and Scale includes 2.5 million for $199 a month.
Subscriber-specific images used to need an engineering project. Now they're a design task, and your list won't know or care how much work happens behind each one.