Our web site at rosskeen.freechurch.org automatically publishes the first few lines from a post onto facebook and a link onto twitter. I personally never go on Facebook and so was unaware that this automatic process was grabbing an image from the website and placing it next to the entry in Facebook.
The image it has been grabbing recently is an image in the footer of the website. The image is related to Donate via Give.net
I have now added two more images to the footer, though on the web site they are set to be invisible they are present and Facebook should pick one of the two rather than the current image.