Make sure the email format is set to HTML

The most common reason for images displaying as attachments is that some messages are sent in the plain text format instead of the HTML format. As the plain text format does not support embedding or viewing images, all images are automatically attached to the message.

Automatic format change can happen either because of Outlook or Exchange Server settings:

Outlook

In Outlook 2010, 2013 and 2016 you can set all messages to be sent in HTML by default.

Just go to File > Options > Mail > Compose messages and set Compose messages in this format option to HTML.

 

For Outlook 2007, go to Tools > Options > Mail Format. Then, from the list which is next to Compose in this message format, choose HTML.

Remember that in the case of replies and forwards, the default message format is the same as in the original message. As a result, if you want to respond to a plain text message, you have to change the message format manually by clicking Format Text tab on the ribbon and choosing HTML.

 

If you compose a message in the reading pane, be sure to click Pop Out button first:

 

Note that in Outlook there is an option to receive all messages in the plain text format. The side-effect of setting this option on is that all replies and forwards are also in this format by default. If you want to know how to turn this option off, go to The receiver converts all messages to the plain text format.

On-premises Exchange Server and Office 365

Your Exchange Server might convert outbound messages to the plain text format. It can happen on every on-premises Exchange Server, as well as on Office 365 with Exchange Online. By default, all messages are converted to the HTML format, unless they are originally in the plain text format – in which case they continue to use simple text formatting. However sometimes, especially in Office 365, some HTML-formatted messages are converted to plain text anyway. As it happens on the server level, it cannot be influenced by Outlook. Luckily, there is a PowerShell cmdlet which lets you change this Exchange policy quickly:

Get-RemoteDomain | Set-RemoteDomain –ContentType MimeHtml

This cmdlet should work on all emails sent outside the organization, converting them to use HTML formatting.

The receiver converts all messages to the plain text format

Another option is that it is the recipient’s fault. MS Outlook and some security programs may convert all messages to plain text automatically. If it is Outlook, above the message there will be a note that “This message was converted to plain text.” The message can be converted to HTML format by clicking the Infobar and choosing Display as HTML:

 

Automatic email conversion can be turned off in Trust Center settings under E-mail Security tab, Read all standard mail in plain text, by checking off the checkbox:

 

However, if you are the sender of the message, you do not have much influence on those settings on the recipient’s side.

Match the HTML display size and the actual image size

First of all, Outlook is known for having a unique way of interpreting HTML code. One of the effects can be displaying images as attachments, and not showing them in the message. This may happen if the width and height parameters of an image are not the same as actual image size. Because of that, you have to make sure that your HTML email signature does not have such differences. Ensure your pictures have the right resolution and resize them if necessary (You can use any image editor, like Paint, GIMP, or Photoshop). Then, add the width and height tags corresponding to the dimensions of the image.

How to edit the HTML file of an email signature

Email signature editor in Outlook does not let you modify the source HTML code. There is an easy way to edit Outlook email signatures, though:

1. If you have a signature created in Outlook, go to mail options and click Signatures… while holding the Ctrl key:
2. A window with your defined signatures will open. To edit the one you want, right-click it and choose a simple text editor like notepad. (For your convenience, you could also use a more advanced code editor, like Notepad++).
3. Now all that is left is to find your img tag and change width and height attributes, if necessary.
   

Linked images are sent as embedded

This problem occurs mostly in the older versions of Outlook (Outlook 2007 and Outlook 2010), but can also happen in Outlook 2013 and 2016 if some settings were migrated, or if someone changed the Outlook configuration.

Usually, linked images should not become attachments – that’s one of the differences between linked and embedded images (More differences in Images in email signatures – linked or embedded?). However, sometimes Outlook automatically downloads linked pictures and embeds them in the message. To change that behavior, you have to make some changes in the registry.

Note: Be careful when editing the registry, as it is easy to harm your computer or programs you use if you are not sure what you are doing. Be sure to always backup your registry before applying any changes.

1. First, start Regedit with “Win + R” key combination and typing in Regedit.
2. The path you need to access is:

HKEY_CURRENT_USER\Software\Microsoft\Office\X\Outlook\Options\Mail

Note X stands for the Outlook version you have, e.g. if you have Outlook 2016, it would be 16.0

 

When you access this location, you either have to find or add Send Pictures With Document REG_DWORD key and change its value to 0.

Thanks to that, the pictures will not be downloaded by your Outlook before the message is sent.

A different approach

If you find troubleshooting troublesome (and, by definition, it usually is), you could take a different approach, which makes images display correctly in email signatures every time. Images are not blocked by the recipients’ Outlook (unless they view emails in plain text only), nor are they displayed as attachments. The solution is a third party tool for Office 365 or Exchange Server.

Regardless of the platform, email signatures are added on the server level, which renders issues with email clients irrelevant.

You can add images with a single button and decide whether you want to embed them, or add them as linked pictures (see this article for differences between linked and embedded images). Either way, the images will not be displayed as attachments.