Yes, those are exact name conventions I've used. I've even tried jpg pics, bmp pics, png with transparent background pics, even gifs, 8 bit, 16 bit, 24 bit, 32 bit, web pallette, normal palette - in Gimp, or photoshop, even online converters, etc. Nothing works. Always some issues.
EDIT: I've tried 256 colors (8-bit) PNG with transparent background, and that's the only combination which works! Finally
Thanks for your help! This was tough, much more complicated than I thought and I would have never figure it out without your help