Alexandru Vornicescu

If you want to resize featured images (post thumbnails) on the fly for your posts automatically then this plugin can help you.
To take effect just open a post or page and all featured images in that post will be resized to a normal size.
This plugin can be very useful after a theme install.
It can be an alternative for Regenerate Thumbnails plugin.
I think this plugin is better because you will not need to wait before all images are resized for all different sizes, just for images that are need to resize.
The plugin will keep the original copy of all image files.

Resize Post Thumbnails

This plugin is very useful and time-saving! Works fast and doesn't need any extra steps.

Best alternative to Regenerate Thumbnails

This plugin will resize post thumbnails on the fly.

Bytes.co

gschoppe

WP SmartCrop allows you to set the ‘focal point’ of any image, right from the media library, then crops your images to match your css rules (no more squished aspect ratios), keeping that focal point, not only in view, but as near as possible to a ‘rule of thirds’ position in the image. The results are powerful, composed photos at any screen size, with any constraints.
<ul>
<li>Visually select your images’ focal points with an intuitive click-based interface</li>
<li>Quickly enable or disable smart-cropping on a per-image basis.</li>
<li>Easily target only the images you want to style, with the img.wpsmartcrop-image selector</li>
<li>Automatic image preloader, so your users never see partially loaded images again</li>
<li>Semantic HTML5/CSS3 for modern browsers, with automatic fallback to non-semantic HTML4/CSS2 for older browsers.</li>
<li>Lightweight and fast processing of images, even inside post content</li>
<li>Fully compatible with common caching and optimization plugins and solutions, such as W3 Total Cache, Wordfence, WPEngine’s EverCache, Better WordPress Minify, and Google’s PageSpeed Module.</li>
<li>Lets you take full advantage of the new responsive srcset features added in WordPress 4.4, to make the most of your images.</li>
<li>NEW! Legacy support for existing cropped thumb sizes allows you to see improvements, right out of the box, without changing your CSS!</li>
</ul>
<h3>Compatibility</h3>
WP SmartCrop has been fully tested in the following browsers:
<ul>
<li>Safari</li>
<li>Chrome</li>
<li>Firefox</li>
<li>Microsoft Edge</li>
</ul>
This is by no means an exhaustive list of supported devices; it just represents our primary target platforms.
<h4>Please Rate/Review and Enjoy</h4>
Your Ratings and Reviews help us to direct our development, and help other users know what to expect.

WP SmartCrop

This function should be a core functionality! So when I found this little gem I was so happy.

A small but significant improvement to WP

WP SmartCrop will crop your images on-the-fly to match your CSS, keeping the main focal point in view.

Resize Post Thumbnails vs WP SmartCrop