[Select and cut out part of image in Affinity Designer]

I am moving to designer from Fireworks and still very new, and I cannot seem to figure out how to do this one thing, which does not appear in the tutorials. I have a picture and I want to select a portion of it, then remove everything else except the elected part. In Fireworks, I could create polygonal or freehand selections and then cut or copy the selected portion to 'disconnect' it from the larger image. I see and have tried the 'Freehand Selection Tool' in Designer, but the selection does not seem to do anything. What am I missing?

 

Thanks a lot,

 

B