When Zabbix imports a map including nameless icon, the icon name becomes "3".
Please see attached files.

[How to reproduce]
At first, I made a map like "map_label_01.png".
I exported the map and imported to another Zabbix.
Then, the imported map became like "map_label_02.png".

The nameless icons in "map_label_01.png" became named as "3" in "map_label_02.png".

[Why it occurs]
The default value of icon is set as "3" in "frontends/php/api/classes/class.cmap.php" line 879.
I suggest to set the default value as ''.
I attached a patch as zabbix-1.8.4-fix_importing_map_icon_label.patch.