@PoweRDeaD
after investigation our code and sample document, I see that reason of the issue is in concatenation matrix operator in the start of the page contents:
1 0 0 -1 0 841.889764 cm
q
…
i.e. all page contents is rotated (including image you added).
You could add GSave operator (q) in the start of the page contents and GRestore after its end, something like that:
page.Contents.Insert(1, new Aspose.Pdf.Operators.GSave());
page.Resources.Images.Add(imageStream);
page.Contents.Add(new Aspose.Pdf.Operators.GRestore());
or update your matrix according yo the page contents rotation.
I’ve created ticket PDFNET-53568 for more detailed investigation of the issue.