Remove wordpress image on facebook

[Paul11]

how do i remove (or change) the image that comes up as the thumb nail when posting a photogrpahy site on facebook (and other places)

it seems to choose one of the images from the site to use/ which i dont want

are these called gravatars ?

 

[Paul11]

thank you

yes it is my site

ill have a look at the plugins

its been driving me crazy trying to solve it