Check uploaded image's dimensions

    Image img = System.Drawing.Image.FromFile("test.jpg");
    int width = img.Width;
    int height = img.Height;

You may need to add the System.Drawing reference.

You may also use the FromStream function if you have not saved the image to disk yet, but looking at how you're using the image (viewable by user in an Image control), I suspect it's already on disk. Stream to image may or may not be faster than disk to image. You might want to do some profiling to see which has better performance.

In ASP.NET you typically have the byte[] or the Stream when a file is uploaded. Below, I show you one way to do this where bytes is the byte[] of the file uploaded. If you're saving the file fisrt then you have a physical file. and you can use what @Jakob or @Fun Mun Pieng have shown you.

Either ways, be SURE to dispose your Image instance like I've shown here. That's very important (the others have not shown this).

  using (Stream memStream = new MemoryStream(bytes))
  {
    using (Image img = System.Drawing.Image.FromStream(memStream))
    {
      int width = img.Width;
      int height = img.Height;
    }
  }

Try the following:

public bool ValidateFileDimensions()
{
    using(System.Drawing.Image myImage =
           System.Drawing.Image.FromStream(FileUpload1.PostedFile.InputStream))
    {
        return (myImage.Height == 140 && myImage.Width == 140);
    }
}