Users, who work with PowerPoint Images have reported better results if they Export to TIFF from PowerPoint (that was added about 2 - 3 PP versions ago), and then Scale (in Ps, or PsE) those to match their Premiere Project's Settings, regarding Frame Size, and then Save_As either TIFF, or PSD, with no compression.
While many people do not see the degradation in the JPEG compression results, but some do.
You might want to explore that workflow with 1 - 2 PP Images (I'd choose ones with the most data in them), to see if that improves the results.
Good luck, and hope that gets your Images into the Project with better output.
The conversion from PowerPoint to video rarely works well -- particularly with text. It's usually best to recreate the text in the Premiere Elements titler.
That said, make sure that your project is properly set up. When you start your Premiere Elements project, go to Settings and select the preset for Full HD 1920x1080. (Or, if you're creating a video for DVD, select DV Standard 4:3.)
If your slides are going to be used in an HD project, make sure that you've sized your JPEGs to 1080 pixels tall. If your slides are going to be used in a DVD project, make sure that you've sized your JPEGs to 480 pixels tall.
Then, before you import your JPEGs into Premiere Elements project, go to the Preferences (under the Edit menu on a PC) and uncheck Scale Still Photos to Frame size.
And remember to render your video (by pressing Enter) before you play it back on your timeline!