Found the Problem but not able to solve it. It seems that rtMedia puts the Image in an HTML link. easyfancybox is not able to open that so I have to change that link into a regular img link.
Any Ideas how to do so?
EDIT – please prior this Edit Text and forget about the first Input:
It would be also good, or maybe even better, if i disable the lightbox. In this Case i come to the Single Image Site and there I wnat to make the Image clickable so my Lightbox (easy Fancybox) can show the Image.
I think i have to edit the rt-template-functions.php to do so but I am not sure what to change there. Maybe its anyway the wrong spot 
I hope you can help me out with that. Would be truly kickass!
btw: Masonary View is still not working. Not sure what the Problem there is but like I said, thats not the Main Problem.