Ok, so digging deeper into the issue it turned out it was not even wordpress nor its themes/plugins (wordpress did blurry the image a little, but not as horribly as it was being shown in the Facebook thumbnail), it was instead the Facebook compressor that does not get along with big spaces of a single color (like the text and the background).
I solved the problem by using Textures instead of colors.
Thanks for the quick response nonetheless as it helped me get in the right direction. I know a lot of wordpress plugin’s forums where it is impossible to get even 1 answer.
