Using different image providers

Hi there,

I´m having problems using a different ImageProvider for ribbon buttons.
Here is my xaml code:

<ribbon:PopupButton
                    shared:ImageProvider.Provider="{x:Static common:ImageProviders.NonDefaultImageProvider}"
                    ImageSourceLarge="{Binding DesignThumbnail.ThumbnailStream128X168, Converter={phx:ImageSourceConverter}}"
                    ImageSourceSmall="{Binding DesignThumbnail.ThumbnailStream16X16, Converter={phx:ImageSourceConverter}}"
                    Label="{Binding DisplayNameDependingOnPatternManagement}"
                    ToolTip="{Binding Translation[RibbonSelectedSinglePatternTooltip]}"
                    VariantSize="Large"/>

Is there something I´m doing wrong or is there another way of doing it?