如何将拉伸图像的像素位置映射到原始图像？

My image is 20x20 pixels and when it's loaded it stretchs to fill the space of the grid which is WxH. When I click on the image I collect the mouse locations. Later I use them to draw the dots on the 20x20 image and save it to the file. But, because the image was stretched, I need to map the mouse location on WxH image to my original 20x20 image.My image is 20x20 pixels and when it's loaded i