Linked image: no hover effekt when using PNG ;(

Hey aebli,
Sorry for the late reply, Please try this code in the General Styling > Quick CSS field or in the WordPress > Customize > Additional CSS field:

#top.page-id-218 .all_colors .image-overlay-inside{display: block !important;}

#top.page-id-218 a.avia_image:hover .image-overlay.overlay-type-extern {
left: -5px!important;  
top: 0px!important;  
overflow: hidden!important;  
display: block!important;  
height: 100%!important;  
width: 100%!important; 
}

After applying the css, Please clear your browser cache and check.

This css is set for your one page in the Private Content area so to not conflict with other pages, also I notice that some of the logos have a smaller height than the overlay icon, I recommend resizing your images so they are square.

Best regards,
Mike