Now it works! What happened before was the following:
Put image box on the stage
Select image X
Save and generate app file
Later you decide to change image X to image Y
Save again and generate again app file.
After closing Mobi One and re-opening it the image X showed up again instead of image Y
I tried it several times and only after deleting the image box could get rid of the old image X.
Now I tried it again and it works perfectly.