PowerPoint supports a custom startup template. This file is called Blank.potx, and is located by default in the UserTemplates folder defined in Word. By default the folder will be %appdata%\Microsoft\Templates.
To have a proper preview displayed when using a custom Blank.potx in the start screen introduced in Office 2013 and later, you need to check the box that says Save Thumbnail when saving your custom made Blank.potx. As such:
Go to the Info pane:
Click Properties, Summary and check the box for Save preview picture:
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