PDF3D SDK PRO contains PDF3D Exporter class API. Within there is:
virtual bool
DrawImage (PDF3DRect drawRect, int imageWidth, int imageHeight, unsigned char *data, bool isRGBA=false, DrawFormatType drawFormat=eJpegFormat, double jpegQuality=0.85f)
virtual bool
DrawImageFromFile (PDF3DPoint drawPosition, const char *fileName, DrawFormatType drawFormat=eAutoFormat, double jpegQuality=0.85f, bool forceRepack=false)
virtual bool
DrawImageScaleFromFile (PDF3DRect drawRect, const char *fileName, DrawFormatType drawFormat=eAutoFormat, double jpegQuality=0.85f, bool forceRepack=false)
with
PDF3DRect (double left, double bottom, double width, double height)
The units for PDF3DRect is in points, which are 1/72 inch per point. This defines the LOCATION and SIZE on the PDF page. It has nothing to do about actual image pixel resolution. You could have a postal stamp size image with 2k x 2k pixel resolution if you wish.
So if you use the simple DrawImageFromFile method, or have put image size pixel height and width values into PDF3DRect width and height, you will get 72 pixels per inch.