You can add a logo/image in you signature by following the steps given below:
To add an image/logo to your signature
1. Go to
Setup (

)
>
Personal Settings >
Signature and click the
Edit icon.
2. In the Signature popup, enter your signature details.
3. Click the Insert Image icon if you want to select an image.

4. Select the image URL or browse for it from your files.

5. Click Insert.
6. Click Save.