For a tileset, you would determine how big your want your tiles to be, then make an image that can fit 16 tiles in each direction. For example, 16x16 tiles mean you would use a 256x256 image.
Each of those tiles, in order, replaces one of the ASCII characters in the game. The easiest way to see which is which is to look at one of the ASCII tilesets in Data\Art for comparison. Alpha transparency is used for the background color, so you will want to use an image program that supports that, like Photoshop or GraphicsGale Free. Tiles that display things defined by the raws can be moved by changing the raws to match what you want; everything else is hard coded. When this is finished, but the image in Data\Art.
Once you have that image set up, you will need to modify init.ini to use your tileset.
Open Data/init/init.txt and make these changes:
Change FONT and FULLFONT to list the filename of your tileset.
Change WINDOWEDX and WINDOWEDY to the width and height you want the game to be in windowed mode. These should be a multiple of the width and height of your tileset.
Change FULLSCREENX and FULLSCREENY to match the resolution of your tileset. FULLSCREENX should be the tile width * 80. FULLSCREENY should be the tile height * 25.
To distribute it, you'll want to create a ZIP file with the tileset, init.txt, and any raw files you've changed. You should keep the original folder structure to make installation easier.