Picture.BorderWeight

public double BorderWeight { get; set; }

[C#]
//Instantiating a Workbook object
Workbook workbook = new Workbook();
Worksheet worksheet = workbook.Worksheets[0];
//Adding a picture at the location of a cell whose row and column indices are 1 in the worksheet. It is "B2" cell
int imgIndex = worksheet.Pictures.Add(1, 1, "example.jpeg");
//Get the inserted picture object
Picture pic = worksheet.Pictures[imgIndex];
//Set the border color of the picture
pic.BorderLineColor = Color.Red;
//Set the border width of the picture
pic.BorderWeight = 3;
//Save the excel file.
workbook.Save("result.xlsx");