As shown in the attached image,even I only calcuate those 4 points image of a square and connect those 4 points image in the simulation. The real image is still in trapezoidal shape.
But the center point  in the object is not corresponds to center point in the image (check out the second image-a snap shot image from the following URL).

Please check out [url=]object in front of concave/convex lens[/url] for more detail calculated image. You can dra any point in the object and find out the corresponding image points.