You could read in the image as a bitmap (the constructor has an overload that takes a file name), then draw that image into a new Bitmap:

public Bitmap ResizeBitmap(Bitmap b, int nWidth, int nHeight)
{
    Bitmap result = new Bitmap(nWidth, nHeight);
    using (Graphics g = Graphics.FromImage((Image)result))
        g.DrawImage(b, 0, 0, nWidth, nHeight);
    return result;
}

You would then add it to the document through whichever element you're using, such as:

section.AddImage(new Infragistics.Documents.Graphics.Image(bmp));

-Matt