By default WordPress doesn’t allows you to crop your post’s featured images (thumbnails). This tutorial is on a simple WordPress plugin: Crop Thumbnails which enables powerful image cropping functionality in WordPress. With this plugin you can crop images directly from post admin (add new & edit) pages using same old featured image metabox.
Other than this, from its options page present in Dashboard -> Settings -> Crop Thumbnails you can also select for what post-type what sizes should be visible in the plugin interface.
Installation & Usage: Install and activate Crop Thumbnails plugin.
How To Crop Post Thumbnails ? After activation you can start cropping your post thumbnails by clicking the image crop button in featured image meta box present in add new & edit post pages.
How To Crop Images Attached In Your Post ?
The plugin adds one image crop button in visual editor of WordPress. Using this button you can crop images attached in your blog post.
All images attached with your post will show up in lightbox overlay, you can choose the image you like to crop.
How To Crop Images From WordPress Media Library ?
Another crop button is added integrated in the WordPress Media Library.