Shape.Rotation Property

workbook.BeginUpdate()
Try
    Dim worksheet As Worksheet = workbook.Worksheets(0)
    ' Insert picture from the file.
    Dim pic As Picture = worksheet.Pictures.AddPicture("Pictures\x-docserver.png", worksheet.Cells("B5"))
    ' Specify rotation angle.
    pic.Rotation = 30
    ' Increase rotation angle.
    pic.IncrementRotation(15)
Finally
    workbook.EndUpdate()
End Try

workbook.BeginUpdate();
try
{
    Worksheet worksheet = workbook.Worksheets[0];
    // Insert picture from the file.
    Picture pic = worksheet.Pictures.AddPicture("Pictures\\x-docserver.png", worksheet.Cells["B5"]);
    // Specify rotation angle.
    pic.Rotation = 30;
    // Increase rotation angle.
    pic.IncrementRotation(15);
}
finally
{
    workbook.EndUpdate();
}