You can install the latest version of 3DSteroid / 3DSteroid Pro from Play
store.
3DSteroid
https://play.google.com/store/apps/details?id=jp.suto.stereoroid
3DSteroid Pro
https://play.google.com/store/apps/details?id=jp.suto.stereoroidpro
Ver3.23(2018/05/04)
You can select the internal SD card or the external SD card for opening image files.
DOOGEE 3D Y6 Max 3D / PPTV King 7S / Support Commander (Freevi) 3D Tablet
(If you have pseudo viewing problem in 3DLCD mode , change settings "Swap L/R for 3DLCD" to "Odd Line as left Image" and that is checked.)
(3D液晶での表示は、ステレオ形式選択で3DLCDを選択する。左右が逆視になる場合は、設定で、「3DLCDの左右入替」をチェックする。)
How to select the storage.