The code I added to FlashAsset.ascx.cs

Alternate asset

if (va.AlternateAssociatedLibraryAsset != null)
{
    if (!String.IsNullOrEmpty(va.AlternateAssociatedLibraryAsset.FileName))
    {
        if (va.AlternateAssociatedLibraryAsset.FileName.StartsWith("/images/", StringComparison.CurrentCultureIgnoreCase))
            this.altfilename = va.AlternateAssociatedLibraryAsset.FileName;
        else 
            this.altfilename = ApplicationSettings.AssetLibraryImageFolder + va.AlternateAssociatedLibraryAsset.FileName;

        this.altlink = va.HrefLink;
    }
}