Ok c:
1. Go find your image that you want to you.
2. Right click on the image. If you're using Chrome browser, select "Copy image address".
3. Open the signature editor on CS.
4. Click the button that says Img.
5. [img ] and [/img] will pop up. Paste the link you just copied in between the two of them.
6. Hit preview, and you should see your image show up.