Images in email signatures – linked or embedded?

Images in email signatures – linked or embedded?

Famous 'box with red x' indicating issues with an image
Fig. 1. Famous ‘box with red x’ indicating issues with an image.

Here is what you need to know to make an informed decision between using linked or embedded images in email signatures (and emails in general):

Embedded images

Sent as hidden email attachments* and referenced in the source of the message using a unique Content ID:

<IMG src="cid:unique content id">

Pros and Cons

tic-yes displayed automatically when the email is viewed
tic-yes can be viewed when the email client is offline
tic-yes not affected by external factors
tic-no can be blocked by antivirus software
tic-no increase the size of emails
tic-no add paperclip icon to messages in OWA

* it is also possible to embed images in emails as text strings encoded using the Base64 scheme, but tests show that most popular email clients will not display them (see this article for more)

Linked images

Located on e.g. a web server and downloaded into the message each time it is viewed by the recipient. Referenced in the source of the email using their web URL:

<IMG src="https://www.mail-signatures.com/wp-content/uploads/2013/07/7454211E-24A9-48b2-969D-4F93B679618F.jpg">

Pros and Cons

tic-yes cannot be blocked by antivirus software
tic-yes do not increase email size
tic-no not displayed automatically in most popular email clients (recipient must click a ‘download images’ button or change settings for specific sender)
tic-no recipient has to be online
tic-no affected by external factors (web server down, accidental deletion, URL change)

Suggested reading

Learn more about using images in HTML
Image types available in popular email clients and enterprise mail platforms
How to rotate banners in company mail without the help of IT
How to unify email signatures in company mail