
Mod makers have the possibility to create custom icons for units. See the wiki for more info: http://taspring.clan-sy.com/wiki/CustomUnitIcons
Please read that Wiki page before posting questions here.
You can grab the dev build to test it out here: http://taspring.clan-sy.com/dl/spring_0.73b1_dev.exe
Note that thanks to the new Spring Data Dir feature you don't have to pack your mod everytime you made a change; just place it unpacked in /mods/yourmod.sdd/ .
You can grab a set of test icons here:
http://teake.home.fmf.nl/temp/unit_icons.zip
If you want to change the distance at which units become icons, you'll have to add an extra DWORD "UnitIconDist" to \HKEY_CURRENT_USER\Software\SJ\spring. The default value is 200 (in decimals). Don't worry, this will be added to settings.exe in the next release :P.