Moving this discussion to the InDesign forum.
PDF and placing would be the correct method.
You said its image, what kind of image(JPG, JPEG, PNG, BMP) ?
You don't have to create a PDF, but it would be helpful to know what, exactly, you want to do with the ppt.
You can export as a PDF which will keep vectors and text as PostScript (crisp) elements.
You can also Save As from ppt in a variety of formats such as PNG or TIFF.
These rasterize all the vectors and text, but are very useful if you're laying out a miniature gallery of the powerpoint presentation.
However, if you want to make the InDesign file a replica of the ppt slides for print, PDF is probably the best way to go.