As per my understanding, you are using the legacy Aspose.Pdf.Generator approach for Image to PDF conversion. However we recommend you to please try using new Document Object Model for Converting an Image to PDF. Please take a look over following code snippet which is similar to the one which you have shared above.
Please note that when you do not specify page dimensions (Width/Height), the image will be placed in center of page and page orientation will be Portrait by default. However when placing images with larger width as compared to Height, there will be some issues with image visibility. So in order to resolve such issues, its better to specify the page dimensions equal to Image dimensions.
Furthermore, in order to remove the white space around page border, you can update/set the Page margin information. Please take a look over following code snippet and in case you still face any issue, please share your image files, so that we can test the scenario in our environment.
[C#]
Aspose.Pdf.Document pdf = new Aspose.Pdf.Document();<o:p></o:p>