Microsoft, in their wisdom, cripple PowerPoint’s abilities to export slides to high resolution graphic files, and by default they compress images imported to a mere 220 pixels per inch or less. Here are a couple of tricks to improve things.
1. Tell PowerPoint not to automatically compress imported images.
Open PowerPoint.On the File menu choose Options. In the options dialog choose Advanced. In the right panel under Image Size and Quality select Do not compress images in file.
2. Change the export resolution for PowerPoint slides
This step involves editing the registry. The registry is a database maintained by the system of all the relevant settings that allow programs to work. If you muck up the registry, things might not work afterwards. If you are unsure, get expert assistance. That said, it isn’t difficult if you follow the instructions here: https://support.microsoft.com/en-us/help/827745/how-to-change-the-export-resolution-of-a-powerpoint-slide
Since this change is done in the registry, it will be in operation whenever you use PowerPoint from then on. Note that if you upgrade to a new powerpoint, the registry change may not be copied to the new version so you may need to repeat the process after each version upgrade.
Note that no matter how high you set the resolution in the registry PowerPoint will not export slides as image files with sides greater than 3072 pixels – that is usually enough, but may not be for some applications. The next section has an approach that gets around this limitation.
3. Alternative — export by printing to a PDF file
This is less convenient but does not need changes in the registry. Print the slide (or slides) using a PDF writer. WIth Adobe’s PDF writer you can set a high resolution (the default is to compress large images to 150 ppi). In the print dialog with Adobe PDF selected choose Properties
If you don’t have adobe PDF writer, there are lots of alternative free programs such as Foxit PDF, or CutePDF Writer which can give high quality output.
Once you have the slide(s) saved as a PDF file containing embedded high resolution images you can save them as image files from Acrobat (paid version) or open them using programs such as (expensive) Adobe photoshop or Adobe Illustrator, or (free, open source programs) GIMP (sim to photoshop) or Inkscape (sim to Illustrator), from where you can export to whatever graphic file format you require.