see I have tested your code it's working, but it image and graphics need to dispose\
String ext = Path.GetExtension(fileAttach.PostedFile.FileName);
string fn = DateTime.Now.ToString("yyyyMMddHHmmssffff");
int width = 760;
int height = 760;
Stream stream = fileAttach.PostedFile.InputStream;
Bitmap image = new Bitmap(stream);
Bitmap target = new Bitmap(width, height);
Graphics graphic = Graphics.FromImage(target);
graphic.DrawImage(image, 0, 0, width, height);
graphic.Dispose();
image.Dispose();
target.Save(Server.MapPath("~\\Site-Content\\Uploaded_Photo\\" + fn + "" + ext));