Adding a image to an expansion tile - flutter

Question

I want to add an image as a leading widget to my expansion tile. Every entry of my List has a different image. I added the image path to every entry of my list but the image is not displayed.
This is my List:
class Entry{
final String title;
final String imageUrl;
final List<Entry> children;
Entry(this.title, this.imageUrl, [this.children = const <Entry>[]]);
}
final List<Entry> data = <Entry>[
Entry(
'title', "/assets/images/picture.png",
<Entry>[
Entry(
'subtitle', '/assets/images/picture.jpg',
<Entry>[
Entry('subtitle', '/assets/images/example.jpg' ),
Entry('subtitle', '/assets/images/example.jpg' ),
Entry('subtitle', '/assets/images/example.jpg' ),
],
),
],
),
];
this is the code of the expansion tile:
import 'package:flutter/material.dart';
import '../models/entry.dart';
class EntryItem extends StatelessWidget {
const EntryItem(this.entry);
final Entry entry;
Widget _buildTiles(Entry root) {
if (root.children.isEmpty) {
return Padding(
padding: const EdgeInsets.all(10.0),
child: Container(
decoration: BoxDecoration(
borderRadius: BorderRadius.circular(15),
border: Border.all(
width: 2,
color: Color.fromRGBO(3, 120, 163, 1),
),
),
child: ListTile(
leading: Image.asset(root.imageUrl),
title: Text(
root.title,
),
),
),
);
}
return Padding(
padding: const EdgeInsets.all(10.0),
child: Container(
decoration: BoxDecoration(
borderRadius: BorderRadius.circular(15),
border: Border.all(
width: 3,
color: Color.fromRGBO(3, 120, 163, 1),
),
),
child: ExpansionTile(
key: PageStorageKey<Entry>(root),
leading: Image.asset(root.imageUrl),
title: Text(root.title, style: TextStyle(color: Colors.black)),
children: root.children.map<Widget>(_buildTiles).toList(),
),
),
);
}
#override
Widget build(BuildContext context) {
return _buildTiles(entry);
}
}
adding the image path directly to the leading widget works but displays the same image for all tiles.
Thank you in advance.