Are you using one of our themes and Twitter/Google+/Facebook/insert-social-site-here grab the wrong featured image from your site (or don’t grab an image at all)?
Most social services use something called OpenGraph to grab content from your site. This is how they know to read the correct title, text and featured image from your posts. And when they don’t find this data they try to parse the source of your site and grab whatever they believe is relevant (such as the first image, even when it may not be relevant to the actual post).
We’ve considered adding code in your themes for serving OpenGraph data, but this is a grey area as far as WordPress rules are concerned. To receive approval into the WordPress directory, among other things a theme should NOT integrate plugin functionality.
Fortunately, this functionality is already included in existing plugins, one of which we found to work perfectly with our themes – WordPress SEO by Yoast – which we highly recommend (even though they pay us no money 😉 )