The U Default Picture plugin is filling empty images fields with
a fail-back image straight in the database at onContentBeforeSave
- If the article's category have a defined image the that image image is used as fail-back image in article's images fields.
- If the article's category does not have a fail-back image, a global, site-wide image is used as fallback image in article's images fields.
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.
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.