It appears there is a bug in the Banner skin object that was likely introduced around 5.6, where you can't use the BannerTypeId attribute in your skin. Here is the offending code:

Dim BannerType As Integer
If AllowNullBannerType Then
    If Not String.IsNullOrEmpty(BannerTypeId) Then
        BannerType = Int32.Parse(CType(BannerTypeId, String))
    End If
Else
    If String.IsNullOrEmpty(BannerTypeId) Then
        BannerType = PortalController.GetPortalSettingAsInteger("BannerTypeId", PortalSettings.PortalId, 1)
    End If
End If

 If you don't set AllowNullBannerType (which defaults to false) and you DO set BannerTypeId, then the BannerType int doesn't get set. The good news is there is a workaround by setting AllowNullBannerType="true" if you want to set BannerTypeId, but I don't think that was the original intention for that property. Hopefully this helps someone; it broke banners in my site after upgrading (good thing I upgraded on a test site first!)

I wish I could, but there are some issues I have to address in some old custom modules before I can upgrade my client to version 6. It looks like the bug still exists in 6.1.2, though:

int BannerType = 0;
if (AllowNullBannerType)
{
    if (!string.IsNullOrEmpty(BannerTypeId))
    {
        BannerType = Int32.Parse(Convert.ToString(BannerTypeId));
    }
}
else
{
    if (string.IsNullOrEmpty(BannerTypeId))
    {
        BannerType = PortalController.GetPortalSettingAsInteger("BannerTypeId", PortalSettings.PortalId, 1);
    }
}
 

 If AllowNullBannerType is false, then BannerType ONLY gets set if BannerTypeId is NOT set (unless I'm totally reading this wrong!) Maybe you could change the code above to:

 int BannerType = 0;
if (AllowNullBannerType)
{
    if (!string.IsNullOrEmpty(BannerTypeId))
    {
        BannerType = Int32.Parse(Convert.ToString(BannerTypeId));
    }
}
else
{
    if (!int.TryParse(BannerTypeId, out BannerType))
    {
        BannerType = PortalController.GetPortalSettingAsInteger("BannerTypeId", PortalSettings.PortalId, 1);
    }
}

 That way BannerType gets set to the value passed in IF it can be parsed as an integer, otherwise it gets set to the PortalSetting value. Regardless, we can still work around it by setting AllowNullBannerType=true if we want to use BannerTypeId as an attribute so it's a pretty low priority issue... I just figured I'd share the find just in case it helps someone out.