Need to convert pixels into points

Hello i am working on a web application in which user has to select a particular point on image from mouse than from server side, i need to adhere another image on PDF file at the point that user has selected. Now my problem is that when i try to adhere the image using that coordinate i.e. in pixels. Images get inserted somewhere else on PDF file. This is because i am getting user coordinates in pixels and PdfFileMend class consider coordinates in points. I get stuck here, Kindly help.