Why are my images not loading on eBay

Because of this system:

In short, when you first try to access a website hosted with us, you’re presented with some Javascript code that generates a cookie, and only with that cookie can you access your site.

When you hotlink images from elsewhere, you still need that cookie. But for security reasons, browsers won’t execute Javascript if they are looking for an image. So the image just won’t load.

So the reason it worked in Firefox is because you had already visited your website and had a valid cookie. But your iPhone didn’t.