How can I embed an image in an Apple Mail signature?

1. Once the Mail program has launched, open the Mail application Preferences. This can be done by selecting it from the Mail menu bar option or by using the "Command - ," keyboard shortcut, which simply consists of pressing ⌘ + , at the same time.
2. Choose the "Signatures" option of the Mail Preferences window.
3. Add a new signature profile by clicking on the "Create a Signature" button located under the second pane within the Signatures Preferences window. This button is represented by a square containing a plus (+) sign. See accompanying image.
4. In the third pane located within the Mail Signatures Preferences Window, type the text of your email signature. You can also paste your text from another application, like Microsoft Word, Apple Pages or Apple TextEdit. Pasting from another application will maintain the text's font, size and style, if that is something you need or desire. Keep in mind that some fonts may not display properly on the email recipient's computer, depending upon the fonts you use and the fonts they have.
5. Assuming that you have an image that you wish to add to your new signature, just drag the image from its location on your Mac (using the Finder) and drop it in the same pane where you just typed or pasted your new signature text. The image should appear next to or below your text. Make sure to use only web compatible images - JPG, PNG or GIF.

You can simply drag an image file into a signature in Mail.

This will embed the image with in the email so all your future emails will include an attachment.

I sugest using a PNG as they are quite light-weight.