In your tazpkg file, there is an "fs" directory, right? Make a few new directories and put your file there:
usr/share/icons/AAA/BBB/xyz.png
Where AAA is the icon set you want it to be in (Slitaz, Tango, etc.) and BBB is the size of the icon (16x16, scalable for svg, etc.). Tazpkg will then automatically copy it to the directories.