If you are a WordPress Thesis framework user then you might have noted that there is no option to set any Image as a featured thumbnail which is an most popular WordPress 3 feature.Instead Thesis have option to set own post thumbnail and Post Image.The problem arises when some of your plugin searches for the featured Image,for example various nrelate plugins like nrelate Related Content,nrelate Flyout and nrelate Most Popular uses WordPress thumbnails while showing the post teaser,if doesn’t find then it uses built in crappy images which will not be related to your content.so what to do ? here is a solution,simple and easy
1.Go to Word press Dashboard
2.Choose Thesis << Custom File Editor
3.Change the editor to custom/custom_functions.php add these lines and save.
// Add Featured Image Support
add_theme_support( 'post-thumbnails' );
add_image_size( 'related-posts', 100, 100, true );
In above code the thumbnail name is referred as related-posts and it’s size is defined as 100*100.
4.Go edit your Post and Add Media , select new file or select from Media Gallery and below that Image you could see option as Use as featured image click on that.Repeat this step for all the posts that you want to set the Thumbnail.
Now clear cache if you are using any cache plugin such as WP super cache or W3 Total Cache and reload your post,Now you could see your featured image appearing but it will not be in defined size for that you need to Regenerate your Thumbnail Images,for that you need to install plugin called Regenerate Thumbnails.
1.Go to WordPress Dashboard << Plugins << Add new
2.search for he term Regenerate Thumbnails and install the first plugin in that list by simply clicking Install Now
3.Now go to WordPress Dashboard << Tools << Regenerate Thumbnails and click Regenerate All Thumbnails,wait for some time the thumbnails will be generated and refresh your post it will work like a charm.
If you don’t get it comment back and I will help you to fix it.