If your image is saved in your computer then shows in pictureBox, this code will be helpful.
If you got the address of image shown in pictureBox, you can use below code to get the file name.
string address = @"C:\mydir\myfile.jpg";
string result;
result = Path.GetFileName(address);
And if you don't have the address of image in pictureBox, you can use this method to get fileAddress:
PictureBox.ImageLocation();
Then use the Path.GetFileName() method.