We were able to observe this issue in the output Pdf as per your description and as highlighted in the following screenshot.
We have logged this issue in our database for investigation. We will look into it and fix this issue if possible. Once, the issue is resolved or we have some other news for you, we will share it with you asap.
This issue has been logged as
CELLSNET-46046 - Round Images become Square Images in the Output Pdf
We have looked into this issue further and found, it is a New Feature and not yet supported by Aspose.Cells. Once, this feature is implemented by Aspose.Cells, we will update you in this topic.
This is to inform you that we have fixed your issue “CELLSNET-46046” now. We will soon provide the fix after performing QA and including other enhancements and fixes.