Back again,

I have one more problem: as you said John, it works as a charm with satellite images. From 270 KiB I went to ~50 KiB and the quality of the image is very good. I just did:

with ImageMagick:
convert sattelite_tile.png -define jp2:rate=0.1 -define compression=JPEG2000 satellite_tile.jp2

The problem is there where I have overlays (they have transparency) and maps with just a few colors (compared to the sat image): land, water, some lines, roads and so on. In these cases, the resulted jp2 file has a bigger size. From 60 KiB it goes up to 80KiB. 

I read on Wikipedia, quote: "The PNG (Portable Network Graphics) format is still more space-efficient in the case of images with many pixels of the same color". I guess this is the case.

Is there a way to work around this? I would pretty much like to keep a standard and use only jp2, not two formats.

--
Raiz Roland Robert