@LeSScro
I have replaced the loading.bmp file as you said, but nothing happend, it was not enough, still the old starting image.
Then, I found a loading.zip archive in the ux folder and I tried to replace the loading.bmp image there too. (ux\loading.zip\ui_nextgen\res\common\mdpi)
Now everything is ok, I have my own starting image. Thank you.