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.

JS Morisset

Have you noticed that WordPress creates small images that are a bit “fuzzy” – nothing like the nice sharp original you uploaded?
After resizing an image, the image must be sharpened but WordPress doesn’t do any sharpening, so the resized image remains a bit “fuzzy” – not what you want for a featured image or shared image on social sites! The WPSSO Tune WP Image Editors (WPSSO TIE) add-on provides this missing WordPress image sharpened feature – it automatically applies sharpening to all JPEG images resized by the WordPress ImageMagick library.
Compatible with all image compression / optimization plugins:
Image sharpening is applied during the WordPress resize operation so the resulting images can still be optimized with any compression / optimization plugin.

<h3>WPSSO TIE Add-on Features</h3>
Extends the features of the <a href="https://wordpress.org/plugins/wpsso/" rel="ugc">WPSSO Core plugin</a> (required plugin).
Applies adjustments to resized images (aka thumbnails) using ImageMagick:
<ul>
<li>Enable or disable image adjustments / sharpening for resized images.</li>
<li>Uses a better compression quality of 92% (WordPress default is 82%).</li>
<li>Applies a default amount of sharpening values to all resized images.</li>
</ul>
Optionally select different primary / secondary image editor(s) for WordPress:
<ul>
<li>GD Only</li>
<li>GD and ImageMagick</li>
<li>ImageMagick Only</li>
<li>ImageMagick and GD (WordPress default)</li>
</ul>
Optionally fine-tune the image filter priority and image adjustment options:
<ul>
<li>Modify the default ‘image_make_intermediate_size’ filter hook priority.</li>
<li>Enable / disable contrast leveling.</li>
<li>Increase / decrease the compression quality percentage.</li>
<li>Adjust sharpening values individually (sigma, radius, amount, threshold).</li>
</ul>
<h3>WPSSO Core Required</h3>
WPSSO Tune WP Image Editors is an add-on for the <a href="https://wordpress.org/plugins/wpsso/" rel="ugc">WPSSO Core plugin</a>, which creates extensive and complete structured data to present your content at its best for social sites and search results – no matter how URLs are shared, reshared, messaged, posted, embedded, or crawled.

WPSSO Tune WP Image Editors

Improves the appearance of WordPress images for better click through rates from social and search sites.

Resize Post Thumbnails vs WPSSO Tune WP Image Editors