Before you begin working on your bitmap (that is, before starting to lay out individual PDF components), make sure that the bitmap has the target ICC profile. You say that your individual photos have the necessary profile built into them, and this is lost when converting. To save the profile, make sure you use the following download method that matches any color profile information:
http://msdn.microsoft.com/en-us/library/4sahykhd.aspx
You can use this first bitmap as a target for your composition, and then export it later. It should keep the original ICC profile.
Alternatively, you can use the FreeImage or ImageMagick libraries to manage color profiles explicitly. Both of them have .NET wrappers and allow you to delete / apply ICC profile files:
Free image library
Magician Command Line Profile Options
Image of Magick.NET Wrapper
Hope this helps.
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