You can find this in your theme
app\design\frontend\{packagename}\{themename}\Magento_Catalog\templates\category\image.phtml
If you don't find the file here then you can create your own with same name having content:
<?php
$_helper = $this->helper('Magento\Catalog\Helper\Output');
$_category = $block->getCurrentCategory();
$_imgHtml = '';
if ($_imgUrl = $_category->getImageUrl()) {
$_imgHtml = '<div class="category-image"><img src="' . $_imgUrl . '" alt="' . $block->escapeHtml($_category->getName()) . '" title="' . $block->escapeHtml($_category->getName()) . '" class="image" /></div>';
$_imgHtml = $_helper->categoryAttribute($_category, $_imgHtml, 'image');
/* @escapeNotVerified */ echo $_imgHtml;
}
?>
and you can modify it as you want. :)
Magento uses fallback method so this file is initially placed in Magento_Catalog::view/frontend/templates/category/image.phtml
You can overwrite it in your theme.