Yes, it must be in the format.... "yourmapname.img" and of course leave off the quotation marks. It is ok to have spaces in your map name, "Your Map Name.img"
You can have multiple transparent / overlay maps display at the same time. I ocassionally will have a custom vector topo map and a custom vector trail map overlayed at the same time with City Navigator. It is in a very rural area with few roads being displayed in CN, this would not be a good practice in a populated area since the map would be so cluttered it would be hard to tell much about it.
edit 6-7-13: It appears as though the Garmin 530 HCx does not allow you to rename the map files transferred to it. See the post 2 below by Indrid Cold for clairification.
You can have multiple transparent / overlay maps display at the same time. I ocassionally will have a custom vector topo map and a custom vector trail map overlayed at the same time with City Navigator. It is in a very rural area with few roads being displayed in CN, this would not be a good practice in a populated area since the map would be so cluttered it would be hard to tell much about it.
edit 6-7-13: It appears as though the Garmin 530 HCx does not allow you to rename the map files transferred to it. See the post 2 below by Indrid Cold for clairification.