In order to resolve your problem, you may consider setting page dimensions (Width x Height) equal to input image dimensions. Please try using following code snippet.
[C#]
//
load source PDF file<o:p></o:p>
Document doc = newDocument();
//
add page to PDF document
doc.Pages.Add();
//
instantiate Image instance
Aspose.Pdf.Image image = new Aspose.Pdf.Image();
//
specify path for image file
image.File
= "c:/pdftest/iPhone_5s.jpg";
//
set left-top-bottom-right margin for first apge