I find xnethack default tileset rather ugly. Under Debian/Ubuntu (nethack-x11 package), the tileset file must be in XPM format. It’s configuration is in the /etc/X11/app-defaults/NetHack file with the default xpm: /usr/share/pixmaps/nethack/x11tiles.

The file must be in XPM format, with a limited size and different color number, otherwise Nethack loading time explodes (several minutes).

Here is how to change it on Debian / Ubuntu:

1. Example: on http://www.multifoliate.com/nh/, get the chozo32b.zip file (link: NetHack 3.4.2 (32×32) (modified by me)). Extract it to get chozo32b.bmp.
2. Open it with Gimp, cut it to 1280×864 (to delete the black at the bottom). You can then convert it to 256 colors, then save it as XPM. It works but it’s ugly.
3. To get a better result, save it to PNG.
4. Install imagemagick package, then convert -depth 5 chozo32b.png chozo5.png
5. Open chozo5.png with Gimp. Then save it to XPM format.
6. Edit /etc/X11/app-defaults/NetHack and replace /usr/share/pixmaps/nethack/x11tiles with the path to chozo5.xpm.
7. Then, reload X ressources: xrdb -merge /etc/X11/app-defaults/NetHack
8. Start your favorite game: xnethack

The final gzipped file: chozo5.xpm.gz

Notes:

• convert always saves xpms in 256 colors. I don’t know why.
• head image.xpm gives you the first lines of the XPM file, with number of different colors.
• Abigada tileset is by default in NetHack iPhone version.

Links:

• Abigada tiles
• Tilesets on Nethack wiki
• XPM Format Specification