Customizing images in link attachments
If you're sharing to Facebook, LinkedIn or Google+ and you've included a link, you'll see a link preview pop up in the composer. When multiple images are found on the page that you're linking to, you'll be able to click on the arrows to select a different image. And if you'd rather, you can hover over the existing image to upload your own.
One thing to keep in mind is that unfortunately Google+ don't allow customized thumbnails for a link post. Any customized thumbnail will be posted to Facebook and LinkedIn, whilst Google+ will select one automatically for the link you've provided.