As image blocking in email continues to become the norm, one absolute must is to make sure you include the width and height attributes in your image tags.
When most email clients (especially desktop based ones like Outlook) disable images, they show an empty image placeholder in its place. Because these email clients don't actually download the images from the server, the only way they can figure out the dimensions of that placeholder is to look at the included width and height attributes. If none or only one is provided, they just take a guess, which in almost every case results in completely destroying what's left of your design.
Here's a perfect example of this in action. Just this morning I received an email newsletter that only specified the height for most images in the email, and not the width. When Outlook displayed the email, it got the height right, but was way off on the width side. Here's how the email looked when I first opened it:
To see a comparison of how it's supposed to look, here's a screenshot of the email with images enabled:
By not including the width attribute in any image tags, Outlook had no idea what width to use and its best guess was unfortunately way off. This made an otherwise readable email a complete nightmare that was almost impossible to get anything out of.
To provide a comparison, I checked out the source and added the correct width attribute to each image to see what the new results would be. Here's a screenshot of the new version that took about 5 minutes to update:
The updated version that includes all width and height attributes is a big improvement over the initial version. It clearly resembles the intended design and I can easily scan the table of contents and start scrolling to read the rest of the content. The email is completely usable even with no images being visible.
While there are certainly better examples of emails designed to look and work well with images disabled, the point is still very convincing. By ensuring width and height attributes are present for all image tags, we give our subscribers a much better chance of getting a usable email, even with images disabled.