How to tighten gates belt drive on Focus planet 8
It's hard to tell from your pictures, one from the side would be nice.  I did a search and came up with the following picture.



If this is what your bike looks like, then you need to loosen the two bolts I've labeled on both sides (4 total) and slide the wheel back until the belt is tensioned correctly. Make sure the wheel is straight and even on both sides when tightening everything back up. From looking at your pictures, it also looks like you should tighten the bolt on the inside of the chainstay where you've circled in red to prevent the wheel from slipping.