tldr; i have solved the issues.
So to summarize :
I managed to bypass the CORS issue by writing a script wich would convert the images into javascript files that hold huge arrays of the levels (wich are more than 7 times as big as the original images).
Directly including it into the program meant, there was no cross origin request anymore and the game worked from the local directory again.
But as i updated the game on itch.io and reinstalled it through the client it just showed a blank page.
Turns out you have to restart the whole itch.io client, in order to test a new version of your own game (at least with an html5 setup).
Is it only me that had that many problems to get my projects running in the itch.io client, or is no one realy using the client?