Choose a thumbnail for display
When your done writing and editing your post, go down to the Custom Fields tab, and either choose (from the dropdown box) or type in Thumbnail (case sensitive) put the full file path to the thumbnail image you want to display on the front page and in excerpts into the values field.
Remember to click the “add custom field button” and update the post in order to save.
Otherwise the theme will choose the first image in your post (or displayed in the gallery) as the default image for the post.
