How do i control what image shows up in a FB post .

I was posting url links in other peoples FB post and when i did that the image that showed up was the paypal image that is st the bottom of the side bar. Is there a way to control what is seen or changes that i need to make to the site to stop that from happening. This is one of the sites in my network i know how to control it when i start a post but its when i insert it into a post on fb that is already there is when i have the problem. I was hopping that it would pull the Avatar image automatically. Thanks in advance for any feed back