Maybe. As per Microsoft, graphics CANNOT be used in Outlook Web Access (OWA) signatures; however, a number of users report having success by following these steps:
1. From the OWA menu, choose “Options” > “email security” > “install SMIME control”.
2. Insert the image (e.g. GIF/JPG/BMP) into the signature. Make sure you add some text before you copy/paste the image (i.e. Best regards,).
3. Save and close.
4. Compose and email via OWA and check if the image shows up.
Note: This is just a finding and may or may not work in your case.
Also Microsoft’s stand on this is that we cannot have images in OWA signatures, and hence is not a supported configuration.