WordPress themes need thumbnails in different sizes and they even let the user to change those dimensions.
But WordPress has a problem: it does not regenerates the thumbnails when new sizes are registered.
Thumbnails intercepts the request by themes or plugins to WordPress to get a specific thumbnail, generates it and caches it on disk.
Efficiently, it produces a perfectly cropped image, with the right dimension avoiding unpleasant stretching.
Thumbnails does not modify your blog or your media library. When deactivated the blog returns to its old behavior.
The second important feature is the autoselection of featured image for post which are missing it.
You can choose between the on the fly selection without persist it or, for better performances, to persist it once selected.
Read the Thumbnails official page for detailed information.
Theme developers can find instructions to use it (no code tied to Thumbnails needs to be written!).
Other plugins by Stefano Lissa:
Screen shots
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