How can I get the image src?

You can use document.getElementById('imageCodeDisplayId').src to access the src attribute of the img element.

You can use the ExecutScriptAsync() method to retrieve the value via C#.

Here you go:

// get the src attribute value
string srcAttributeValue = await webView21.CoreWebView2.ExecuteScriptAsync("document.getElementById('imageCodeDisplayId').src");

// show it in a message box
MessageBox.Show(srcAttributeValue);

JavaScript code can be injected into WebView2 controls at runtime using host apps. The injected JavaScript applies to all new top-level documents and any child frames. The JavaScript injected into the page is executed at a precise point in time.

You can use the following java script to select src attribute of specified img tag and then show it in an alert:

alert(document.getElementById('imageCodeDisplayId').attributes['src'].value);

So in your WevView2's EnsureHttps use something like the following code:

void EnsureHttps(object sender, CoreWebView2NavigationStartingEventArgs args)
{
    webView.CoreWebView2
        .ExecuteScriptAsync(@"alert(document.getElementById('imageCodeDisplayId').attributes['src'].value);");
    args.Cancel = true;
}

Output:

All extra necessary information is available at Get started with WebView2 in WinForms apps

And also WebView2.ExecuteScriptAsync(String) Method describes everything that you need to use it.