I did it step by step, and after even few own changes it worked.
I'm not sure if the mods are going to delete this thread now that the answer is clear, but it would be useful to me or someone else take a look at here how it was done.
Thanks a lot!
Edit: In case you're wondering, I did those own changes in export settings which about Rainbow Dash mentioned.
Ok, have to remember these 3 things:
1. Must convert adding images to 256 colors
2. Must convert the image itself to 256 colors
3. Must set the export settings right (not let GIMP write color space information) by checking a box
Btw, I know this puts things upside down but I noticed that also Paint screws up the image and because I can't use Paint properly I just use GIMP.
[This message has been edited by Oldroman (edited 06-18-2015 @ 01:56 PM).]