To edit images in FE or any (GBA) game, all you need is a tile editing program. Below, you will find a list of tile editors.

Tile Layer Pro (I use this one, pretty simple and easy)
YY-CHR
Tile Molester

This table shows which images can be found through a tile editor.

FE6 mini portraits
0060DBC0

FE7 Japanese
00C6BF18

FE8 Japanese
00861970 - Beginning of all portraits, including compressed class portraits
0087DB81 - Character portraits (NPC/enemy/ally)

FE8 English
0080D418 - Beginning of all portraits, including compressed class portraits
008295C0 - Character portraits (NPC/enemy/ally)

This basically means you can't access the portraits normally using a tile editor.

Compressed portraits can be found and edited in a program called unlz-gba. You should easily be able to find it via Google by searching "unlz-gba.exe".

Since I only use Tile Layer Pro, I can only provide help for this program. Most other programs are probably similar though, with the basic tools.

First, open your ROM file in TLP. You will see fuzzy tiles, which is data viewed as tiles/images (but since it's not meant to be an image, it just shows up as what it is- fuzziness). To find things, you can go to certain offset values (which I have provided above) or just keep scrolling down. So, go to the offset value, and you will find more fuzzy looking tiles which look a bit more like something.

Now stretch your window with your loaded ROM in it to 32 tiles/squares. If you have a resolution of 1024x768, there is a kind of shortcut. Move your small "Tile Editor" window all the way to the right (or left, if you are more comfortable with it), then stretch the window with your game all the way across so it fills the space next to the "Tile Editor" window as shown here:

You should also change the colours so you can view the tiles better. Change the colours int he "Palette Editor", or you can load an existing palette (here you can find a download for some FE8 portrait palettes). Note: Changing the palette here does NOT affect the colours in the actual game, it is only for viewing purposes. In the screenshot above, I have loaded Tana's (from FE8) palette just to view the portraits.

If you notice in the screenshot, the portraits still look a bit misaligned. Use the buttons + and - to "move the bytes". You can also press "left" and "right" to move the tiles around too. Soon, you will find the portrait looks a bit more aligned, like here:

Now this is as aligned as you can get it to be. This image below shows you how the portraits are aligned:

To edit face sprites, you can check out my other tutorial (here).

Note: It is also important to note that a character's mouth frames are above its portrait. The whole portrait data of a character begins from its palette data which appears as a fuzzy tile before the mouth frames.