I think you just need to use this in your dialogue definition:
<Control Id="BannerBitmap" Type="Bitmap" X="0" Y="0" Width="370" Height="44" TabSkip="no" Text="!(loc.InstallDirDlgBannerBitmap)"/>
When I need to create a user dialog, I just copy one of the standard dialogs from wix sources and adapt it to my needs. All these controls have the above line, and I never had to do anything extra so that the banners in my custom dialogs were the same as for the rest.
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