In the interests of total accuracy and to supplement and reinforce xtrail's excellent post above:
This is the map naming convention for image files which will work in all units nuvi 1xxx to 3xxx series [including your 3590] and some related dezl and zumo models with on-board .System and Map folders and microSD card usable Garmin and Map folders:
In unit's .System folder: gmapprom.img, gmapprom1.img [and for some maps gmapoem.img].
In unit's Map [root] folder: gmapsup1.img and anyname.img.
In card's Map [root] folder: anyname.img.
In card's Garmin [root] folder: anyname.img and gmapsupp.img [that latter name only works there, and it won't work in the unit's Garmin folder, and anyname only works in card's Garmin folder for more recent units, not 1xxx].
There are differing conventions for earlier models such as nuvi 2xx[W]/3x0/465/5x0/6x0/7xx etc and related units which don't have the hidden .System folder and use maps in the unit's Garmin folder. Other relevant threads in here: [Only registered and activated users can see links. ] also [Only registered and activated users can see links. ], and [Only registered and activated users can see links. ] and [Only registered and activated users can see links. ] for earlier units.
One thing that is universal with Garmin devices [automotive, outdoor, marine, aviation] which use a removable media card is that all of them can use a valid map named gmapsupp.img which always works in [card]:\Garmin folder and that's the definitive test for any map image. If it doesn't work there providing the map's suitable for the unit then there's something wrong with the map.