By default save as image file will get you 96 dpi. To change this follow this step:
- Start REGEDIT (Run Regedt32.exe)
- Navigate to HKEY_CURRENT_USER\Software\Microsoft\Office\X.0\PowerPoint\Options (For X> above, substitute 16.0 for PowerPoint 2016, 15.0 for PowerPoint 2013, 14.0 for PowerPoint 2010, 12.0 for PowerPoint 2007 and 11.0 for PowerPoint 2003)
- Add a new DWORD value named ExportBitmapResolution and set its DECIMAL value to the DPI value you want (for example, for a 3000 pixel image from a 10" wide slide, you'd enter 300)
- Close REGEDIT, start PowerPoint and test.
I'm using 250 dpi.