Automaticly copies the image and insert this. The script creates a Layer-Mask and makes a part of the insert layer invisible.
It's great to have a part of the original image on the top, because than you have a very well comparision.
If you have any suggestions for improvement contact me.
I hope this example conform your requests:

Iconify2.scm
Iconify2 plug-in converts a single layer of a single image into a multi-layered image ready to be saved as a Windows icon.
;The new image will contain all standard sizes (16x16, 32x32, 48x48) at all standard bit depths (16 colors, 256 colors, 32-bit RGBA), with transparency support.
*Note: Not fully tested though, so be sure to try it on an "expendable" copy of an image first.
**Tested on - Gimp 2.4.6
- Gimp Portable 2.4.6
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