Fabien Quatravaux

Sometimes, your website design requires to turn pictures in black & white : for example in a gallery display, you can have all thumbnails in grayscale and bring up colors when the mouse is hovering it. 
This plugin will make it possible without the need to manualy create the grayscale variant of each picture.
In your theme’s <code>functions.php</code>, instead of using WP <a href="https://codex.wordpress.org/Function_Reference/add_image_size" rel="nofollow ugc">add_image_size</a> function, use <code>grayscale_add_image_size('custom_size', xxx, yyy, $crop, $grayscale)</code> (with <code>$crop</code> set to <code>true</code> if you want to crop the image, and <code>$grayscale</code> set to <code>true</code> if you want a grayscale version of the image). Then, you can use <a href="https://codex.wordpress.org/Function_Reference/the_post_thumbnail" rel="nofollow ugc">the_post_thumbnail</a> or any thumbnail function to get your grayscale image :
<pre><code>the_post_thumbnail('custom_size-gray')
</code></pre>
A gamma correction is applied automatically to make the black & white pictures look good. From version 1.2, You can tune this gamma correction with the <code>grayscale_gamma_correction</code> filter. Your filter would have to return a float number that will be use as the <code>outputgamma</code> parameter of the <a href="http://php.net/manual/fr/function.imagegammacorrect.php" rel="nofollow ugc">imagegammacorrect</a> function.
<h4>Known bugs</h4>
<ul>
<li>Grayscale images are not generated when the original image size is smaller than the specified thumbnail size.</li>
</ul>

Grayscale

Automatically create grayscale thumbnails for a given image size.

phayoune

Phayoune Site Ribbon is a lightweight, standalone plugin designed to display a fixed floating image (such as a mourning ribbon, promotion badge, or logo) on any corner of your WordPress website.
It works with any theme and does not require Elementor. This plugin is perfect for site announcements, mourning periods (Black & White ribbon), or verified badges.
🔥 New in Version 2.8: 
Advanced Grayscale Control! We have separated the logic for Grayscale (Mourning) Mode. You can now choose to apply the Black & White effect to specific pages or posts only, independent of where the ribbon image is displayed. This gives you full control over your site’s appearance during mourning periods.
🔥 New in Version 2.3: 
We have integrated the WordPress Media Uploader! You can now select your ribbon image directly from your Media Library with a single click. No more manually copying and pasting image URLs.
Key Features: 
* Easy Image Selection: Choose images directly from your WordPress Media Library. 
* Custom Position: Easily place it Top-Left, Top-Right, Bottom-Left, or Bottom-Right. 
* Smart Adjustment: Fine-tune position with X/Y Offset controls. 
* Advanced Grayscale Mode: Turn your website into black & white mode. (New: Supports specific page selection independently from the ribbon). 
* Animations: Add movement to your ribbon (Flip, Swing, Pulse) to grab attention. 
* Display Conditions: Choose to show on All Pages, Only Pages, Only Posts, or Specific Pages/Posts (Multi-select). 
* Linkable: Add a custom link to the ribbon image.

Phayoune Site Ribbon

Easily display a floating ribbon or image on your website corner. Includes Grayscale mode, specific page selection, and Media Library integration.

Grayscale vs Phayoune Site Ribbon