I don't recommend using .png graphics files in PowerPoint; there are known issues there. I would use a program such as Paintshop Pro or Paint.net to convert them to .jpg or .gif format and then re-test and re-save your theme. If it doesn't work with graphics files of that format, your PowerPoint is broken and needs to be repaired.
You can uninstall and reinstall your PowerPoint from CD media if you have it, that will often fix the problem. If it doesn't, you may have a script-virus that often comes in the form of an infected attachment via E-mail. If you get to that point you should scan with your built-in antivirus program and download the free Malwarebytes antispyware scanner from Malwarebytes.org. Detecting a script-virus that is office specific and removing it may then fix that problem. Don't forget to make sure all your personal data on that computer is backed up to external media prior to running the virus repairs, as some types of script-viruses can cause irretrievable data loss upon disinfection.
Good luck, <<<BIGBEARJEDI>>>