The crop mode for images is unfortunately not mature at all in my opinion. Cropping an image in the Sitejet Website Builder is quite an adventure with many bugs. That’s why I actually only use this option when I can use the predefined aspect ratios (1:1 etc.) or want to enlarge the crop in the existing aspect ratio.
I miss the possibility to set my own aspect ratios. That would be a real help that would make this tool much more user friendly.
Another new idea just occurred to me: How about setting the aspect ratio already in the image settings? This would allow you to use the image element with a fixed aspect ratio for collection entries, for example. So, I could always use this option very well! That way you wouldn’t have to use a background image for a similar effect.