[How do I crop imported images (and some other issues)?]

I just downloaded Affinity Photo and I absolutely love it. However, there are some major issues that I have (2018 regular iPad):

- the app is crashing *constantly*. It always crashes while applying a refined selection to a new layer with mask, and right now, I’m editing a larger document and I can’t even move images around without it crashing. Restarted iPad, didn’t help. I will stop using it for a few hours now. Is this normal?

- is there a way to crop (destructively, for performance) images independently from each other? This seems like a super basic function, but I haven’t found a way yet. Can I crop imported stock images? 

- is there a way to mask a layer without using the „refined brush“ tool? (Edit: this was a very basic question and I since found out how to work with masks)

Thanks a lot for your help!