It's pretty simple:
Office 2002
using Microsoft.Office.Core;
using PowerPoint;
ApplicationClass pptApplication = new ApplicationClass();
Presentation pptPresentation = pptApplication.Presentations.Open("myfile.ppt", MsoTriState.msoFalse,
MsoTriState.msoFalse, MsoTriState.msoFalse);
pptPresentation.Slides.Item(1).Export("slide.jpg", "jpg", 320, 240);
Office 2003
using Microsoft.Office.Core;
using Microsoft.Office.Interop.PowerPoint;
ApplicationClass pptApplication = new ApplicationClass();
Presentation pptPresentation = pptApplication.Presentations.Open("myfile.ppt", MsoTriState.msoFalse,
MsoTriState.msoFalse, MsoTriState.msoFalse);
pptPresentation.Slides.Item[1].Export("slide.jpg", "jpg", 320, 240);
Image Output Quality
pptPresentation.Slides.Item[1].Export("slide.png", "PNG", 1024, 768);