I am ok for multi-layered .png files for setting custom background. Maybe it is better to use some small additional file that will be loaded as background soil texture? And it should have limited size and color palette to prevent space consuming issues.
wkIndiMask already is solving semi-destructability of the maps... only problem with that is that makes map-making process a bit complicated...

If i understood this already exists as a wormkit module: http://worms2d.info/WkIndiMask

not so much of a problme if we have map cache

Only problem is the memory / loading times required for multiple layers. I would suggest a 1bit/pixel mask for destructablity and a smaller tile for the background like the game itself uses, this would take much less space than having a second full size png, but would remove the option of background that shows even on an undamaged map.

"Sorry if I miss something but...How to add new layers to the indexed .png file?"
The standard PNG file has 1 frame(image) per file.
They are other, animated png file types, but I dont think theirs an official one yet.

My idea for this suggestion is that a second layer of the map can be created by the author, where he can specify destructible and indestructible terrain on the same map.

Sorry if I miss something but...How to add new layers to the indexed .png file?

I am totally for this. And that is exactly how it should work.

Custom background..

Check picture: http://img213.imageshack.us/img213/5926/mapbackgroundla7.png

On a destructible terrain game the soil has a background, visible when you blow the soil up.

Could you make the game load a custom map and background, two different png files with the same attributes?