The text and image used should be coming from the Open Graph tags that are set on the page. Those tags are usually set by JFBConnect automatically. If there's an Open Graph plugin for the component on that page, that will help generate even better tags (for Joomla Content, K2, JomSocial, etc). If you haven't already installed the Open Graph plugins appropriate for your site, that may help.
To specifically set the tags for a page, you can set the data by adding the following tags into the page that is being "sent":
{SCOpenGraph description=Description for the page}
{SCOpenGraph image=http://yoursite.com/link-to-image}
Those tags can be added anywhere in the page (article content, custom module, etc). Once set, it can take Facebook a few days to 'see' the changes, but you can use the
Facebook URL Debug Tool
to test the changes immediately.
Hope that helps,
Alex