Don't try to convert it to a string!
What you get from a DB for an Image is an array of bytes - which the DGV will display as an Image.
To use that as an Image in isolation, you need to convert the bytes to an Image via a MemoryStream:
byte[] bytes = (byte[]) myDataGridView.Rows[rowAddr].Cells[cellAddr].Value;
MemoryStream ms = new MemoryStream(bytes);
myPictureBox.Image = Image.FromStream(ms);