I tried 37x37 and 43x43 images of a circle, but had the same problems as dtjoa.

I fixed the problem by using a custom draw filter class.  Set the UltratoolbarsManager.Ribbon.DrawFilter to an instance of this AppButtonFilter, and add the desired image for your logo in the Resources (or supply it some other way)

<code lang="vb.net">

    Public Class AppButtonFilter
        Implements Infragistics.Win.IUIElementDrawFilter

        Public Function GetPhasesToFilter(ByRef drawParams As Infragistics.Win.UIElementDrawParams) _
        As Infragistics.Win.DrawPhase _
            Implements(Infragistics.Win.IUIElementDrawFilter.GetPhasesToFilter)
            If (TypeOf (drawParams.Element) Is ApplicationMenuButtonUIElement) Then
                Return Infragistics.Win.DrawPhase.BeforeDrawImage
            Else
                Return Infragistics.Win.DrawPhase.None
            End If
        End Function

        Public Function DrawElement(ByVal drawPhase As Infragistics.Win.DrawPhase, _
                                    ByRef drawParams As Infragistics.Win.UIElementDrawParams) As Boolean _
                                    Implements Infragistics.Win.IUIElementDrawFilter.DrawElement
            Dim elementRect As Rectangle = drawParams.Element.Rect
            elementRect.Inflate(-1, -1)
            drawParams.DrawImage(Resources.RedCircleImage, elementRect, True, Nothing)
        End Function
    End Class

</code>

Here are the results:

using ApplicationMenuButtonImage: 

using the Draw Filter:

The images used by the ApplicationMenuButton are 43x43. There are 3 pixels of transparency on all four sides of the circle part of the image. This means the circle actually measures 37x37. If you create a similar image, I believe your image should completely obscure the default image.