Masking images in Sketch is not great. 
Unless I’m missing something, we need to always have a mask object under the image if we want to have full control over the image cropping. I’m aware image can be used as a background of an object but it is too difficult to adjust in most cases.
Masks should be smarter groups. When object is masked I should be able to immediately change radius of the group and when I resize the group it should not stretch everything, but only change the bounds.
The other solution might be adding cropping options to image fills but, at least in my case, I usually don’t work with fills, instead I place images within Sketch as that gives me more options when exporting etc.
Thanks for the suggestion