For use this tool the ROM must read samsung image resources, perhaps it can save them also to a different format to ease the theming process, Img2Png can convert successfully the following formats:
So just download the app, install, and pick an apk from sdcard and the app will extract the qmg resources and convert them to png format and saving them in the Img2Png folder in the sdcard. The output folder retains the folder structure of the extracted apk including drawables and mipmap folders. The tool can't convert sucessfully .9.qmg, the tool only will convert it as normal png files so need to research or keep the original .9.qmg in case that the app use this format
Note: Use at your own risk and no guarantees that it will function as intended on your device, all credits goes to @xperiacle for part of the code of his awesome tool.