Я понял.
string str1 = @"C:\Users\prince\Desktop\PRINCE-PC 11-12-2553 14.07.57\SlideImages";
string str = @"C:\Users\prince\Desktop\PRINCE-PC 11-12-2553 14.07.57\slide.ppt";
Application pptApplication = new Application();
Presentation pptPresentation = pptApplication.Presentations.Open(str, MsoTriState.msoFalse, MsoTriState.msoFalse, MsoTriState.msoFalse);
pptPresentation.SaveAs(str1, PpSaveAsFileType.ppSaveAsJPG, MsoTriState.msoFalse);
pptPresentation.Close();
str1 = OutputFolder
str = путь к файлу PowerPoint