C# PDF Image Editor Library
How to add, insert images to existing PDF file using c# .net

String inputFilePath = Program.RootPath + "\\" + "1.pdf";
String outputFilePath = Program.RootPath + "\\" + "output.pdf";

//  load a sample image
Bitmap anImage = new Bitmap(Program.RootPath + "\\" + "1.png");

//  open the document
PDFDocument doc = new PDFDocument(inputFilePath);
//  get the first page
PDFPage page = (PDFPage)doc.GetPage(0);
//  set image position in the page: X = 100F, Y = 400F
PointF position = new PointF(100F, 400F);

//  add image to the page
PDFImageHandler.AddImage(page, anImage, position);

//  output the new document
doc.Save(outputFilePath);

//  load a sample image
Bitmap bitmap = new Bitmap(Program.RootPath + "\\" + "1.png");

int width = bitmap.Width;
int height = = bitmap.Height;

int scaledWidth = 400;
int scaledHeight = scaledWidth * height / width;

//  set image item option
PDFItemOptions ops = new PDFItemOptions();

//  set target page region to add the image.
ops.Position = new PointF(10F, 10F);
ops.Width = scaledWidth;
ops.Height = scaledHeight;
             
//  open the document
PDFDocument doc = new PDFDocument(inputFilePath);
int pageIndex = 0;

//  add image to the target page
PDFImageHandler.AddImage(doc, pageIndex, bitmap, ops);

//  output the new document
doc.Save(outputFilePath);

String inputFilePath = @"C:\1.pdf";
String outputFilePath = @"C:\1_backgd.b.pdf";
String srcImageFilePath = @"C:\SourceImage.png";

//  open a PDF file
PDFDocument doc = new PDFDocument(inputFilePath);

//  Create a background resource by an image file.
PDFBackgroudRes resBackground = new PDFBackgroundImageRes(new Bitmap(srcImageFilePath));

//  define a page background setting
PDFPageBackground background = new PDFPageBackground(resBackground);
//  Set transparency
background.Opacity = 0.2F;

//  Alignment the background to center.
background.HoriAlignment = BackgroundHoriAlignment.Center;
background.VertAlignment = BackgroundVertAlignemnt.Center;

//  Indicate if use image's acutal size or not.
//  If set to false, it would fill the image to the target page.
//  Default: false
background.EnableAbsoluteScale = true;

//  define page range: to all pages
PageRangeOptions pageRange = new PageRangeOptions();
pageRange.AllPages = true;
pageRange.Subset = PageRangeSubset.All;

//  apply page background settings to all pages
PDFPageFieldHandler.ApplyBackground(doc, background, pageRange);

doc.Save(outputFilePath);

String inputFilePath = @"C:\1.pdf";

//  Open file
PDFDocument doc = new PDFDocument(inputFilePath);
//  Get all images in the document
List<PDFImage> imgs = PDFImageHandler.ExtractImages(doc);
foreach (PDFImage img in imgs)
{
    if (!img.IsInlineImage && !img.IsForm)
    {
        //  Get color space name of the image. Eg.: DeviceGray, DeviceRGB, Indexed etc.
        Console.WriteLine("Color Space: {0}", img.GetColorSpaceName());
        //  Indicate if the image is in color (RGB).
        Console.WriteLine("Is RGB:      {0}", img.IsRGB);
        //  Indicate if the image is in grayscale.
        Console.WriteLine("Is Gray:     {0}", img.IsGray);
    }
}

String inputFilePath = Program.RootPath + "\\" + "2.pdf";
String outputFilePath = Program.RootPath + "\\" + "output2.pdf";

PDFDocument doc = new PDFDocument(inputFilePath);

PDFPage page = (PDFPage)doc.GetPage(0);

List<PDFImage> images = PDFImageHandler.ExtractImages(page);

PDFImageHandler.ReduceImageSize(doc, images[0], 2F);

doc.Save(outputFilePath);