How to parse an image tag in xml sitemap?

It is a good idea to familiarize yourself with XML Namespaces first.

I'm going to assume that at a higher level you have something that is explaining what image: represents. If you don't, that's a different problem and you arguably don't have XML, just something that looks a lot like it. Here's you exact same XML but wrapped in what I assume is an Image Sitemap as defined by Google.

$xml = <<<'TAG'
<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9"
        xmlns:image="http://www.google.com/schemas/sitemap-image/1.1">
<url>
<loc>
<![CDATA[ https://www.news18.com/photogallery/photogallery/in-photos-five-most-expensive-face-masks-for-protection-from-covid-19-in-the-world-3519821.html ]]>
</loc>
<image:image>
<image:loc>
<![CDATA[ https://images.news18.com/ibnlive/uploads/2021/03/1615377235_mask-1.jpg ]]>
</image:loc>
<image:caption>
<![CDATA[ ]]>
</image:caption>
</image:image>
<image:image>
<image:loc>
<![CDATA[ https://images.news18.com/ibnlive/uploads/2021/03/1615377250_mask-2.jpg ]]>
</image:loc>
<image:caption>
<![CDATA[ ]]>
</image:caption>
</image:image>
</url>
</urlset>
TAG;

To access things by a namespace, you call children() on the node with the appropriate namespace expansion:

$doc = simplexml_load_string($xml);
echo $doc->url[0]->children('http://www.google.com/schemas/sitemap-image/1.1')->image->loc;

This prints out:

 https://images.news18.com/ibnlive/uploads/2021/03/1615377235_mask-1.jpg