The latest CapLab post-release Beta patch v2.6.13 supports for modding natural resource bitmaps displayed on the main city map.
Please read this tutorial on the CapLab web site for details about how to MOD natural resources:
http://www.capitalismlab.com/natural-re ... amond.html
Link for downloading v2.6.13 patch: http://www.capitalismlab.com/forum/view ... 2160#p9852