How to Create a Proper HTML Email Signature for Apple's Mail.app
The above video will show you how to create a proper HTML email signature for Mail.app on OSX. This is particularly important if your signature uses images. If you add an image through Mail's signature preferences pane, the image will be attached to each outgoing message rather than being embedded. This makes your messages bulkier, taking up unnecessary space in your inbox and your recipients inbox, and may make them more likely to trigger a spam filter.
Open Mail's preferences, and add a new signature.
Open a Finder window.
Select "Go" in the menu bar and press the option key to make the Library option appear. Click Library to open the library folder in a Finder window.
Navigate through the following folders: Mail, V2, MailData, Signatures
Locate the appropriate signature file. The signature files consist of a string of numbers and characters followed by the mailsignature file extension.
Open the signature data file in a text editor.
Delete everything after the MimeVersion line, and replace it with the HTML code for your desired signature.
Save your changes, and close the file.
Right click the signature file in Finder, and select "Get Info."
Lock the file by selecting the check box next to "Locked."