It's easy to create an Outlook signature. Just click on the Signature button in a new message, select Signatures and then click New and start typing. The only problem with this approach is that the signature editor is fairly limited.
A better option is to use the new mail message itself. You'll have access to a wider range of formatting tools and, best of all, Picture Tools if you're including an image:
For instance, you can quickly resize, crop, and even apply Artistic Effects:
When you're done designing your signature, copy it to the clipboard, open the Signatures tool, and simply paste in your masterpiece: