Why people care to play the GBA dragon ball ? games I will never know (even by anime game standards they leave a bit to be desired). They also have some oddities that make them a bit of a pain to hack compared to most other games on the GBA, though you might get lucky here.
Anyway is that just a different colour but otherwise identical sprite? If so that is more likely a palette hack in your future than a sprite hack and your sprite sheet will do not a lot for you.
Portraits being changed would be more likely to be a sprite (maybe BG) hack so there is that.
If you can make a sprite sheet I will assume you know it but for the sake of others that land on this from a search.
Most console games don't note the colours of their sprites (the little things that move around on screen, sometimes called objects aka objs as indeed they are handled on the GBA by a section of memory called the object area memory aka OAM) or their backgrounds (BGs, commonly used for backgrounds, larger images and text, though game devs can and frequently do whatever they like and make big images or text in sprite form).
Instead it is all a big paint by numbers affair with the sprites only being a number to look up on a palette. The GBA has software defined palettes which can change all the time (indeed changing a palette is one way animations can be achieved) where some older ones have a fixed list baked into the hardware.
A game might also recycle old sprites but give them new colours by changing the palette, hence the terms palette swapped enemies or palette swapped player characters.
The thing where each pixel is also the colour it represents is known as a bitmap image/format. Some consoles have these, however they are not commonly used as they are slower to do things with (they are larger in size)
Find the palette in the game (can be as simple as dumping it from memory when the game is being played and searching for that data in the ROM -- palettes are usually small and not compressed, and while you can animate palettes it is not that common and you can usually still search for the chunk that is not animated), edit this to reflect your new colours (most tile editors will have the option to load, edit and save palettes) or indeed possibly copy and paste the colours from the end game bonus mode or whatever that is and you have your hack.
Be aware a game might use the same colours for something else on the screen, or later in the game somewhere else and changing colours can then break something else or change something else somewhere else.
If this is the case then you get to also alter the sprites (some of which might be reused elsewhere so be aware of fallout from that). Finding sprites can be harder but you can also try dumping and searching, or loading up a tile editor before setting the right mode (GBA 4bpp and GBA 8bpp) and pressing down a lot to scan the whole ROM (takes some time but still within reason), you might also try compression searches. Tile editors will also help here. Backgrounds are edited much the same way, though you might have bitmap options here.
To replace portraits you will have to find the ones in the game you want to edit, find the ones in the original game (or maybe dump them from an emulator), overwrite if you can or maybe deal with compression if there is compression involved (
http://members.iinet.net.au/~freeaxs/gbacomp/#BIOS Decompression Functions covers things, and has downloads to GBAcrusher which I like for this).
I cover some of how to do this in my GBA and DS ROM hacking document
https://gbatemp.net/threads/gbatemp-rom-hacking-documentation-project-new-2016-edition-out.73394/
If it is a hard one you might have to do tracing.
Also a guide that might help
https://web.archive.org/web/20080408022631/http://etk.scener.org/?op=showtutorial&st=3
If you want technical explanations
http://problemkaputt.de/gbatek.htm#gbalcdvideocontroller
http://www.coranac.com/tonc/text/video.htm
https://www.cs.rit.edu/~tjh8300/CowBite/CowBiteSpec.htm#Graphics Hardware Overview
