Bro, in all honesty you need to slow down with these constant requests everywhare. We've already answered your question about how to import tilesets into Mari0 in the several other threads you started, and if you want to use one that's already on the site, there's zero need to get an image-editing program to add it into Mari0 or change it. As long as you don't load it into MS Paint, the tileset you want to use will work, as long as it's in a map packs' folder and is titled "tiles.png".
This thread will help you if you really want to make 100% custom tilesets:viewtopic.php?f=12&t=2955
This thread contains a healthy number of already-made tilesets.viewtopic.php?f=12&t=2437