You can automatically set and display a default Post Thumbnail image (featured image) either the first image attached to a post or you can set default images as thumbnails for specific categories, tags, custom taxonomies etc. Also you can exclude certain posts. Supports custom fields to ease upgrading.
Default Thumbnail Plus
First install and activate Default Thumbnail Plus plugin then follow the given steps:
Step-1 After activation visit your admin area Dashboard -> Settings -> Default Thumb Plus page.
Step-2 Now first it will ask to set default thumbnail that will be loaded if the post has no featured image set.
Step-3 Now click ‘Add Filter‘ button, upload image and use dropdown to set selected image as featured thumbnail of posts from any specific category, tag posts etc.
The plugin provides you several other options:
Use image attachment if available: Automatically use the post’s first available image attachment for the thumbnail. This is useful for older posts that haven’t got a featured image set.
Automatically use first embedded image, use embedded image if available: Automatically use the post’s first available embedded image for the thumbnail. This is useful if you embed external images.
Automatically use first embedded video, use embedded video thumbnail if available: Automatically use the post’s first available embedded video for the thumbnail (YouTube only).
Custom field: Enter a custom field key here, it’s value if set will become the default post thumbnail for that post. The custom field value can either be an Attachment ID, or a link to an image.
Excluded posts: List of posts to be ignored by this plugin. Comma separated e.g. 10, 2, 7, 14
Cache and resize external images: Automatically resize, crop and cache external images & video thumbnails. Recommended option, however doesn’t work with some server configurations.
Enable or disable plugin hooks (for developers only): Filter post_thumbnail_html    Filter get_post_metadata
