Set your signature as a canned response to insert your email signature/footer manually.
As an alternative to adding your signature in Mail Settings>General>Signature, use the Canned Responses lab to insert your signature manually just when you need it. This way, if you send a lot of internal mail, your signature is not always added to annoy colleagues.
If you have the Insert Image lab installed, you can upload an image from your hard drive, which you can't do with Mail Settings>General>Signature.
Open a new email and insert an image (using the Insert Image button)
Select My Computer and Choose File to browse to your hard drive. The image you choose will then appear in the box, click OK when you have the image you want.
To add a link to your signature, drag your cursor to select the image - so it is highlighted in blue. Click the link button and in the field "To which URL should this link refer" paste the url link you want
Add a Kind Regards, xx above the image if you want to
Once happy, click on Canned responses and select New canned response - call it "Signature".
You can also upload your signature image to a domain you host and add the url into a canned response, to do this follow the steps above, choosing Web Address (URL) for step 2.
Tip Update: In the new Compose, Canned Responses can be found by clicking on the down arrow in the bottom right of the Compose window.