Pictures disappear while compiling Android app [duplicate]

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• Using Android getIdentifier() 3 answers

I have finished my first application, that shows 180 minerals with his pictures. I have correctly upload the pictures from the drawable folder with the debug .apk, but when I try to generate the signed/release .apk. The pictures are simply not shown.

I call the pictures at java code:

int id = getResources().getIdentifier("com.example.manu.minerals:drawable/" + mineral.pgn, null, null);
    imgM.setImageResource(id);

And the pictures as said are stored in the drawable folder. What´s happenning at the compilation so they are not shown? Should I do something before compiling the signed apk? Thank you in advance.