I can’t seem to find much documentation on the differences between these 2 classes besides PresentationEx supporting the new pptx file version.
First of all, I would like to inform you that currently we are working on a unified API of Aspose.Slides for Java (already released for Aspose.Slides for .NET) which will have a single class i.e. Presentation class for both PPT and PPTX file types. Hopefully, this version will be available by next month as per the current plan.
Now, to answer your question, PresentationEx class is used to work with PPTX files and also come limited support is available for PPT files as well. Whereas Presentation class is for PPT files. I would request you to please share the PPT file and sample code with us to reproduce the issue you are facing for PPT to Image conversion using Presentation object. We will further check it and get back to you.
Thanks & Regards,