Personally I am not bothered by the portraits but my shortcomings when it comes to art criticism are not unknown to me.
By the way it is usually the European versions of the game that get fiddled with, certainly the music patch is for the Euro version. http://www.romhacking.net/?page=hacks&genre=&platform=&game=818&category=&perpage=20&title=&author=&hacksearch=Go
does not have as much as it does for the likes of the other FF advance ports but hey. You will want the sound patch if you are used to the SNES version as well.
We could go long and discuss the GBA graphics system but there is an easier method in this case.
Assuming you do not have the location of the portraits in the ROM you get to find them. Looking at the tile/background viewers is the first step on that path. After that http://www.romhacking.net/documents/361/
covers the method. Hopefully it is just a copy of some form from memory but you could also face compression, such things are not so bad on the GBA.
Alternatively you might find something simply scanning through the ROM with a tile viewer or try a slightly less involved method than the one linked (compression searching, BIOS logging.....), the one linked will get you the location of the data in question though.
Now if you used the method in the guide you could do the in depth removal thing (you would have had the instructions that deal in the loading of the data). The simple method is the GBA has transparency assigned to whatever the first palette value is, overwrite the portraits with transparent values and though the game will still load that area into RAM it will then have nothing visible to display.
As for what you have done thus far the memory location you mention is in the VRAM and nothing to do with the cartridge/ROM at all ( http://nocash.emubase.de/gbatek.htm#gbamemorymap
). It will probably stay in that location for every boot (more dynamic stuff did not happen until the DS and even there is not so common) but nobody really does N64 style texture hacks for the GBA at this point in time. The tile editors will tend to just show jumbled pixels until such a time as you line it all up -- the GBA does not have a filesystem commercial games used so it is all bundled into one file. It may happen that you get lucky some times and it loads fine (tile editors have default values and ROMs work in certain ways that allows for it to happen often enough) but it is not assured.
For the GBA I like, not all GBA supporting tools support all modes (several great tools for the SNES and co that say they support the GBA lack 8bpp GBA format) but the ones I link in a moment do.
Nice little tool, it has issues with custom tile sizes though. Very few things on the GBA use it.
Its palette options are not as nice as the one above but it supports custom tile sizes and a few very odd/esoteric modes.
TileGGD (not really an editor, a very flexible viewer though)http://code.google.com/p/tiledggd/
Compression capable graphics editing is also a thing. http://www.romhacking.net/?page=utilities&category=&platform=10&game=&author=&os=&level=&perpage=20&title=&desc=compressed&utilsearch=Go
has a few. Crystaltile2 also has limited capabilities here.