code to avoid malicious image being upload in asp.net

Question

1) code to avoid malicious image being upload in asp.net

- actully i want fileuploader using asp.net which upload valid image only..but my fileuploader uploading invalid image like image with malicious code(i.e with extension .exe)means i have check extension and all in my code...but i want simultanous checking of image content,so that hacker will not able to upload malicious image etc..