Creating complex mails with inline images
From MozillaZine Knowledge Base
Include images from files
The HTML format allows to place an image within the message text rather than as an attachment. Neither of these methods work in plain-text composition mode, thus verify that the top-most box is checked in Tools → Account Settings → (account) → Composition & Addressing. There are several ways how to do this. This section describes including an image from a local file, whereas the next section describes inserting an image located at a remote server.
Browse for an image
Drag & drop images
You can also drag the image file directly from your desktop or an open folder into your message-composition window to place it there. By default, no alternate text is specified. If you want to include an alternate text, double-click on the image in your message to make the respective settings in the Image Properties dialog.
Copy & paste images
You can copy any image or the content of any open window to the clipboard of your operating system, then paste it into the message. Compared to using the "Insert" menu or drag & drop method, pasting will usually involve recoding of the image, thus may change quality and size. Thus, the other two methods are preferred to retain the original format of an image.
After complaints that Thunderbird 2.0 pastes images in a low-quality JPEG format (see "Hack" below to increase the quality if you are still using 2.0, though it's no longer supported), Thunderbird 3.x now pastes in lossless PNG format by default instead. This substantially improves the image quality for screen shots from applications, but increases the size for photographs which were initially encoded in JPEG. To allow pasting in JPEG, change the clipboard.paste_image_type preference from its default 1 to 0. Note that the setting of 2 to prefer pasting in GIF is not supported on Windows, and depends on the copying application if it is made available on other platforms.
Hack: As a workaround for Thunderbird 2.0 on Windows, and if you feel comfortable modifying programs, you can increase the pasting quality with a hex editor (for example, XVI32). Go to Thunderbird's installation directory. Make a backup copy of the main Thunderbird executable (for example, thunderbird.exe on Windows), then use the hex editor to edit the original. Search for the Unicode string
Note: On Linux, copy & paste or drag & drop may not work for Thunderbird 2.0, or paste a file path as text instead (this is fixed in 3.0).
In an HTML-formatted message, you can send mail with a pointer to a picture rather than embedding the actual picture in the message. The advantage is that the message is much smaller. The disadvantage is that the picture is physically on another server; if the image is unavailable—now or in the future—your recipient will not see it. Some e-mail clients might also be configured to block remote images.
Drag & drop images
Rather than copy-pasting the URI of an image within a web page manually, you can also drag the image directly from a web page you are viewing with Firefox or SeaMonkey into your open HTML message-composition window to place it there. By default, the image will be attached to the message before sending. If you want to include a reference to the remote image only without attaching it, double-click on the image in your message and uncheck the respective box in the Image Properties dialog.
If you create your HTML message in some other application, and insert it in the message by using Thunderbird's Insert – HTML... dialog, then you can usually use that other application to add the moz-do-not-send attribute to each IMG tag. For example, the resulting tag might look like:
<img src="http://static.mozillazine.org/common/images/blimp.png" moz-do-not-send="true">
Images from other messages
You can drag & drop images that are part of (or attached to) a message which you are currently displaying to a new message you are composing.
Drag into message body
Place as attachment
Images referred over CSS
If you create a new HTML-formatted message and use external URIs for images, Thunderbird will automatically download them and attach them as inline images. However, that does not work with images referred over CSS, meaning the absolute URI stays. The problem with this is that some mail clients (including Thunderbird itself) will provide the recipient with a security/privacy warning that external images will be excluded instead of displayed inline.