Well, what I did:
1. save a picture to your computer.
2. click "browse" in signature editor and select you picture.
3. Click "upload".
4. Right click and copy the image that appears.
5. Right click and paste the image into the signature box.
6. save signature.
Hope it works!
By the way, if you want to send a specific person a message, you have to "view conversation" or go to the other person's profile. Then they will be notified and not have to check your profile.