AOL, Hotmail, Yahoo!, Gmail, Outlook 2003, and Thunderbird are blocking HTML images from loading because of increased focus on security and privacy. This impacts the way your message appears in your recipient's mailboxes and therefore also impacts your open and click-through rates.
On top of this, these systems don't work exactly the same. Most leave a blank space where an image would be. In Yahoo!, the space is gray; Gmail displays a broken image icon. Some, such as Gmail and Thunderbird, will also display alternate text if it's provided. Others don't. AOL not only blocks image loading but also disables hypertext links.
In his article "When E-Mail Images Don't Load" on the Clickz website, Derek Harding explains how you can optimize your recipient's viewing experience despite all this variability:
- Examine and optimize how your message appears with no images, so that it still conveys your message and brand in the correct way (eg. through the sender address and/or subject line.
- Make sure to use a descriptive text in the alt-tags for all images - even if not all email clients will show it, at least the ones that do will give your readers an idea of what the image is about.
- Remind your readers to add your email address to their address book to ensure proper displaying of your images.
- Provide a link to an online version of your email.