Compositing/croping question

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Using version 2.8 still but can upgrade if it will make things easier:

I am trying to add though bubbles to pictures and have found a nice bubble png I can put on top of my pictures and that works fine.



The next step is to insert another picture into the bubble, so I drop the thought image on top of the bubble to create a new layer:



Now I need to crop the thought image to the central area of the bubble image. I can use the color select tool on the bubble layer to get the cropping area but I cannot work out how to crop the thought layer using that selection to trim the legs in this image to the size of the bubble. As soon as I switch to the thought layer the selection area changes.

Any help appreciated.