you shouldn't do that without permission of the original creator of the tileset. you can do this with my tilesets though (aztec.bmp, cs2dnorm.bmp, dust.bmp)
many creators of tilesets say that you are allowed to use their tiles in whatever way you like, as long as you give them some credit. Only thing is to find those tilesets...
And then there's still sites offering HUGE amounts of free tiles which are (sometimes a little editing is needed) perfectly usable in CS2D.
DC , this son of the ... stolen my Zombie Skin in Crossfire 2D and post here: http://unrealsoftware.de/files_show.php?file=2488
This is 4 zombies skin class i made and upload before his post in here: