In order to edit the map sprite in the statsheet of a unit, you need to follow these steps--

1. Decompile Fire Emblem DS ROM.
2. Go to Data->2->Class Name
3. The "palette" file can be edited via a hex editor and contains the palette. Palettes are 0x200 bytes and are divided into two parts (not sure why there are 2 palettes or what each one does at the moment, haven't bothered to test). The 'idle' file contains the graphical data for the idle map sprite in the statsheet.
4. By copying/pasting the files of one thing to another, you can literally switch the map sprites up.  Alternatively find a guide in this editor on LZ77-11 decompression and how to edit LZ77-11 compressed graphics so that you can actually alter the graphics.