Using Base64 encoded images will not bypass image blocking in email clients.
It was a known technique used by spammers, and therefore no better than linking to remote files. A test from 2008 by Ron Blaisdell of the Email Marketer's Club (available here), showed the results of sending an email containing Base64 encoded images in the popular clients:
- Gmail: Displays only alt text
- Hotmail: Displays a grey square, not the image, and no alt text
- MS Live Hotmail: Same as Hotmail
- Outlook 2003: Display a broken image, with alt text displayed
- Outlook 2007: Same as Outlook 2003
- Yahoo Classic: Displays only alt text
- New Yahoo Mail: Same as Yahoo Classic
- AOL: Displays alt text
- Gmail: Displays alt text
- Thunderbird: Displays alt text
- Outlook Express: Displays broken image, with alt text displayed
One reliable way to bypass image blocking is to become a trusted sender by asking the receiver to add your email address to their address book.
But there are no widespread hacks or workarounds to force an email client to display images, and any such hacks are likely to be patched quickly. This is one reason why it's important to minimise the number of images you use in email newsletters, and to make sure the main message and call to action is text-based
Update October 2012:
Google Bulk Sender Guidelines now say that Gmail will automatically show images for senders who have authenticated their domain:
To ensure that Gmail can identify you:
- Use a consistent IP address to send bulk mail.
- Keep valid reverse DNS records for the IP address(es) from which you send mail, pointing to your domain.
- Use the same address in the 'From:' header on every bulk mail you send.
We also recommend publishing an SPF record, and signing with DKIM or
By authenticating, inline images you send will be shown automatically. Recipients will not need to click the "Display images
They also offer a page to learn more about email authentication.