@Matt-Nelson In Monaca projects, you can only make changes within www folder. For this reason, you need to use the cordova-plugin-copy-resources plugin to make changes to project files outside of www folder. These changes will be effective after the build. In your case, please do as follows:
Add cordova-plugin-copy-resources plugin into your project.
Within config.xml, change the following line:<resource-file src="/res/android/drawable-mdpi/ic_stat_onesignal_default.png" target="res/drawable-mdpi/ic_stat_onesignal_default.png" />
to<resource dest="res/drawable-mdpi/" src="www/images/ic_stat_onesignal_default.png" />
Note: You need to put your images source files within www folder such as images folder.
This should fix your build issue.