If you want the mask to work correctly, please try to avoid antialiasing it.
A 50% opacity red pixel is not indestructible.
If you are working in GIMP, I would suggest disabling AA in selection, and before saving, use either:
Image->Indexed (enter a number of colours that matches the non-transparent colours your mask uses) then Image->RGB again (since all game PNGs must be RGBA).
You can fix partially translucent existing pixels by reselecting (non-AA select) then flood fill, by indexing the image, or by semi-flattening the layer then fixing the now-white pixels with posterise, erasure or indexing.
Once you've saved it, you can verify the colours using a histogram tool.
$ identify -verbose share/hedgewars/Data/Maps/Lonely_Island/mask.png | grep -A3 Histogram
1710969: ( 0, 0, 0, 0) #00000000 none
139664: ( 0, 0, 0,255) #000000 black
246519: (255,255,255,255) #FFFFFF white
Here, you can see that all the pixels are either black, white or transparent.
If you have some that don't match one of the rules, that pixel will be ignored in the mask, and will just inherit the default value from map.png.
As well as avoiding unexpected behaviour, a PNG with fewer colours compresses slightly better.
Another thing that can help PNGs compress better is the choice of transparent colour.
If you want transparent black (or transparent white) in GIMP, just make a new layer that is black or white, then ctrl-a, ctrl-x erase it, position it below your main mask layer and merge down.
This can save quite a bit of space. For example, I just tidied up the TrophyRace mask.
Before, it was using this transparent "colour"
3212815: (217,215,196, 0) #D9D7C400 rgba(217,215,196,0)
4399210: (255, 0, 0,255) #FF0000 red
5827: (255, 0, 0,254) #FF0000FE rgba(255,0,0,0.996078)
(also notice 5827 non-solid red pixels, that would be ignored)
3869995: ( 0, 0, 0, 0) #00000000 none
4420309: (255, 0, 0,255) #FF0000 red
Reduction in size?
PNG went from 233,882 down to 45,252.