Problem adding a larger layers

[dinasset]

I did the following:
- open a (small) image
- open a (larger) image (whatever, not necessarily a frame)
- duplicate the larger image (to leave the original as it is)
- edit copy the small image and paste as a new layer into the duplicated larger one (automarically gimp set it at centre)
- add alpha channel (transparency) to the top small layer
- apply layer to image size for the top layer (you get the full margin around as transparent)
- do whatever you like on the bottom larger layer (filling, modifying colours, brightness...I just increased the contrast)
You will see all your modifications appear thru the transparent part of the top
When satisfied, flatten the image.
I don't know whether this what was you were asking about.

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PS: you may do in the reverse order the combo steps:
- duplicate the small
- add the larger as a new layer (you will not see its full size at the moment)
- move the added layer below
- apply "canvas to image size"
- add alpha to the smaller layer then layer to image size
......