Hope this helps:

1. From the Tools menu, select Options.
2. Click the Mail Format tab.
3. Click the Signatures button at the bottom right of the window.
4. In the Create Signature window, click the New button.
5. Type a name for your new signature, ensure Start with a blank signature
is selected, then click the Next > button.
6. In the Edit Signature window, type and format any desired text. Right
click in a blank area of the signature window and select Insert Image.
7. In the Picture dialog, click on the Browse button. Navigate to the
desired image and double click to select it. Click on the OK button.
8. Click on the OK button, then use the Signature for New Messages drop down
menu to select the new signature as default. Click on OK to finish.