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I need to display all the sub categories under every category as a grid in relevant pages with linked images. Is there a way to do it ? I am using magento 2.2.4

You need to copy the below file:

vendor/magento/module-catalog/view/frontend/templates/category/products.phtml

to your theme like below:

app/design/frontend/[Vendor]/[theme]/Magento_Catalog/templates/category/products.phtml

And add the below code to product.phtml:

<?php $category = $block->getCurrentCategory(); ?>
<?php $subcategories = $category->getChildrenCategories(); ?>
<?php $_helper = $this->helper('MagentoCatalogHelperOutput'); ?>
<?php $objectManager = MagentoFrameworkAppObjectManager::getInstance(); ?>
<?php if(count($subcategories) > 0): ?>
 <div class="products wrapper grid products-grid">
 <ol class="products list items product-items">
 <?php foreach($category->getChildrenCategories() as $subcategory): ?>
 <?php $subcategory = $objectManager->create('MagentoCatalogModelCategory')->load($subcategory->getId()); ?>
 <li class="item product product-item">
 <div class="product-item-info">
 <?php if ($_imgUrl = $subcategory->getImageUrl()): ?>
 <a href="<?= $subcategory->getUrl() ?>" class="product photo product-item-photo">
 <span class="product-image-container">
 <?php $_imgHtml = '<img src="' . $_imgUrl . '" />'; ?>
 <?php echo $_imgHtml = $_helper->categoryAttribute($subcategory, $_imgHtml, 'image'); ?>
 </span>
 </a>
 <?php endif; ?>
 <div class="product details product-item-details">
 <strong class="product name product-item-name">
 <a class="product-item-link" href="<?= $subcategory->getUrl() ?>"><?= $subcategory->getName() ?></a>
 </strong>
 </div>
 </div>
 </li>
 <?php endforeach; ?>
 </ol>
 </div>
<?php else: ?>
 <?php if (!$block->isContentMode() || $block->isMixedMode()): ?>
 <?= $block->getProductListHtml() ?>
 <?php endif; ?>
<?php endif; ?>

You need to manage the css according to your requirement and also need to hide the layered navigation through css or update it to 1 column according to your requirement.

If you are still looking for a solution to show Subcategories on parent category page. Have a look at Advanced Subcategory Grid module on Magento2 Marketplace that can be used to show subcategories on category pages, its highly customizable and also supports configurable color swatches as-well.