I am afraid, we require your sample spreadsheet to properly evaluate your presented scenario on our end. It would be of great help in isolating the problem cause if you can also share your environment details such as Operating System version/architecture and Target .NET Framework version.
We were able to observe this exception by executing the following code with your source Excel file using the latest version: Aspose.Cells for .NET (Latest Version) . Shape to Image Error occurs on converting Excel file to PDF.
We have logged this issue in our database for investigation. We will look into it and fix this issue. Once, the issue is resolved or we have some other update for you, we will let you know asap.
This issue has been logged as
CELLSNET-43119 - Shape to Image Error on converting Excel file to PDF