I have just written a plugin to scale an image to the size of game screen with/without keeping the ratio. And I have put the script to GitHub over here:
Place it in your project's folder, under js/plugins.
Open your project in RPG Maker MV.
Go to Tools > Plugin Manager.
Double-click an empty line to add a new plugin.
Select the plugin from the drop-down list.
Click the "Event" button from the Toolbar
Double-click the tile that you like to edit event
In the Content list of the right-hand side, double-click the line below "Show Picture" of which you would like to scale the image
Select Tab "3", and click "Plugin Command..."
Fill in the text: stretchPictureFullScreen [number,keepRatio] , in which, number is the number of picture to be scaled. And,if you want to keep the image ratio after scaled, fill in 1 or true in keepRatio, otherwise, fill in 0 or false.
Click "OK" to confirm
Terms of Use
The above plugin is released under MIT License. And it is free to use, repost, and/or modify, for commercial or non-commercial projects.
Credit is required as per MIT License.
A star on GitHub is appreciated but not required.