Thank you for the tips. I will implement it by the next time.

Here is the download link for the test version to load custom levels and graphic files:
https://play.google.com/apps/testing/at.partnercheck.lemmus

To load custom graphics:
1.   Install app
2.   After starting the app and clicking the "Load levels from SD Card" Button the app is creating a "\Lemmus\levels" and "\Lemmus\styles" folder
3.   The graphic .dat files should be copied into the styles folder and the .lvl into the levels folder
4.   After a repeated click on the "Load levels from SD Card" Button all levels will be loaded in alphabetic order and can be played

Copyright is an important topic and to avoid violations the whole code, graphic and sound is designed on my own or my helpers. And many thanks that you allow me to use your graphics.

I have developed a lemming like game for Android. Its name is Lemmus. If anyone is interested in this, I can provide a download link without any levels. But it is possible to load the levels (neolemmix dat and lvl files) from the SD-Card. This project is a free time project and I have developed it on a nexus 5. So there will be some bugs on other smartphones. In the future I will provide it on Google play for free with self-made levels. From time to time the game will show advertising and if you will support me please tab on it. Otherwise you can disable the internet connection and no advertising is shown.