get name of picture?

The image loaded in PictureBox is just an array of bytes, so to find out what is the filename you must fill the Tag property of PictureBox when any image loaded in it.

An Image object only contains the image's binary data. You can manually set the Tag property of the Image to contain the file name (After you've created the image).

If you load an image to the PictureBox using the Load() method, that will update the PictureBox's ImageLocation property to the file's path.

Then you can use pictureBox4.ImageLocation for comparison.

ImageLocation on MSDN