Magento – Magento2: how can I resize the image used in gallery in the product page.

You can check in details by click, Resize Custom Image in Magento 2

Inside Block file keep below code,

   protected $_filesystem ;
   protected $_imageFactory;
   public function __construct(            
        \Magento\Framework\Filesystem $filesystem,         
        \Magento\Framework\Image\AdapterFactory $imageFactory         
        ) {         
        $this->_filesystem = $filesystem;               
        $this->_imageFactory = $imageFactory;         
        }

    // pass imagename, width and height
    public function resize($image, $width = null, $height = null)
    {
        $absolutePath = $this->_filesystem->getDirectoryRead(\Magento\Framework\App\Filesystem\DirectoryList::MEDIA)->getAbsolutePath('custom_module/posts/').$image;
        if (!file_exists($absolutePath)) return false;
        $imageResized = $this->_filesystem->getDirectoryRead(\Magento\Framework\App\Filesystem\DirectoryList::MEDIA)->getAbsolutePath('resized/'.$width.'/').$image;
        if (!file_exists($imageResized)) { // Only resize image if not already exists.
            //create image factory...
            $imageResize = $this->_imageFactory->create();         
            $imageResize->open($absolutePath);
            $imageResize->constrainOnly(TRUE);         
            $imageResize->keepTransparency(TRUE);         
            $imageResize->keepFrame(FALSE);         
            $imageResize->keepAspectRatio(TRUE);         
            $imageResize->resize($width,$height);  
            //destination folder                
            $destination = $imageResized ;    
            //save image      
            $imageResize->save($destination);         
        } 
        $resizedURL = $this->_storeManager->getStore()->getBaseUrl(\Magento\Framework\UrlInterface::URL_TYPE_MEDIA).'resized/'.$width.'/'.$image;
        return $resizedURL;
  } 

Now Call inside phtml file,

$block->resize('test.jpg',500,400);

Magento uses the file called view.xml which is maintained at the theme level of the application.

So for example, if you are using the default theme luma you should find the view.xml under vendor/magento/theme-frontend-luma/etc/view.xml

In this file, you would see node inside the node.

The dimension of the images is maintained here under the node.

The id attribute value of the node is referenced in the code base.

        <image id="product_page_main_image" type="image">
            <width>700</width>
            <height>560</height>
        </image>
        <image id="product_page_main_image_default" type="image">
            <width>700</width>
            <height>560</height>
        </image>

You can define your custom view.xml in your theme and define width & height as per your requirement.

In case of changing/overwriting the values of the view.xml file you need to completely copy the entire view.xml file to your custom theme and change the values.

view.xml does not have a node value fallback system, means if a value of a node is not present in you custom theme's view.xml it will not fallback to its parent theme's view.xml value, that's why entire file needs to be copied.

Once the values changes have been done, you will have to run

php bin/magento catalog:images:resize

This will regenerate the new image sizes.