Failed to perform Web Scraping to the src attribute in img

I'm trying to learn to do Web Scraping, I've already managed to get all the data except one, the src attribute of an image (img), the funny thing is that if I get the alt.

A fragment of the example website:

<li>
<a href="/ver/49688/steinsgate-0-15" class="fa-play">
<span class="Image"><img src="/uploads/animes/thumbs/2950.jpg" alt="Steins;Gate 0"></span>
<span class="Capi">Episodio 15</span>
<strong class="Title">Steins;Gate 0</strong>
</a>
</li>

The fragment of code that does not work:

using System.Net.Http;
using HtmlAgilityPack;
var Ultimo_animes = Ultimos_Episodios[0].Descendants("li")
                .Where(node => node.GetAttributeValue("", "")
                .Equals("")).ToList();
foreach (var Ultimo_anime in Ultimo_animes)
            {
                //Imagen
                Console.WriteLine("Caratula: " +     Ultimo_anime.Descendants("img")
                    .FirstOrDefault().GetAttributeValue("src", ""));
        }

If I change:

GetAttributeValue("src", "")

By:

GetAttributeValue("alt", "")

If you take out: Steins; Gate 0

But with src, it comes out empty

A help please?

Thank you very much in advance and greetings