Marker icon values is related to marker file-name icons, this is important in case if you adding new language, and not add icons, then you should not translate this values. You will see blank/standard markers then.

Google map marker icons can be found in: ‘templates\bootstrap2-responsive\assets\img\markers’

Or if you using realocation template then: templates\realocation\assets\img\icons

You should replace this images with new one or add new like this.

If you want to add new file, name it lowercased and add to field values via administration:
Admin->Real estates->Fields->Add value in list, for example:
empty,commercial,house,land,apartment,your_new_filename

And file your_new_filename.png