You know what is happening when missing images brake your templates layout. The modules that build your homepage are not having a fail-back image and you end up with broken blocks.
Joomla! repository have a few plugins that fill empty article's images with fail-back images. But those are set up at onContentBeforeDisplay event. If the module is set to not call this event then those plugins are not instantiated becoming useless.
And the fail-safe images fields settings from above can be combined or used separately, for Intro Image and Full Article Image, according with the quite self explanatory plugin settings.
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