AddImageFile method of FRDocument object contains PageIndices parameter.
HRESULT AddImageFile(
BSTR ImageFileName,
IPrepareImageMode* PrepareMode,
IIntsCollection* PageIndices
);
use the following C# code to open only 2nd page of an image:
FRengine.IIntsCollection PageIndices = FREngine.CreateIntsCollection();
PageIndices.Add(1);
FRDocument.AddImageFile("MyImage.pdf", null, PageIndices);
Only the pages with numbers added in PageIndices collection will be added to the document.