Just use the "grit" tool included in DevKitPro (located in C:\devkitPro\devkitARM\bin)
In the DevKitPro tile example (C:\devkitPro\examples\nds\Graphics\2D\BackgroundAllInOne) they use
Pern Edit. I think when it comes to combining tiles from multiple maps it can get a bit more tricky. But I'm pretty sure grit can do it anyway. So I say we just use grit for now and figure out how to combine multiple tiles/maps and palettes later.
To create a simple map create a png that is made up of 8x8 tiles Eg. level1.png and a level1.grit containing the following
#-------------------------------------------------------
# graphics in tile format
#-------------------------------------------------------
-gt
#-------------------------------------------------------
# tile reduction
#-------------------------------------------------------
-mR
#-------------------------------------------------------
# map output
#-------------------------------------------------------
-m
#-------------------------------------------------------
# graphics bit depth is 4 (16 color)
#-------------------------------------------------------
-gB4
Grit will create the map data and automatically detect repeated, rotated and mirrored tiles and create the map accordingly.
Here are the full options for grit