I just tried to get this to work, but I can't quite get the hang of it. I tried what you said here first and nothing happened. Then I looked at the source to try to figure out what is wrong, and it seems like the images should be in a subdir called poi/ so I tried that as well, but still nothing happened. It seems like it tries to read the icons from the path stored in _poi_db_dirname and then a subdir called poi/ and then the filename, so I grepped trought the source to see if maybe _poi_db_dirname would be something unexpected.

My C is not too hot, and my gtk is even worse, but I can't find the place where the _poi_db_dirname variable is set in the source. The only code I can find seem to be for changing it after changing it in the settings dialog, but nothing that sets it initially. And sure enough, if I changed the poi db to something else in the settings, and then changed it back to the old value, the icons suddenly work, since that variable is then set properly. But when I restart the program it doesn't go trought the settings dialog, and thus the variable is unset again, and icons stop working.

This should help you fix the problem