Final Fantasy Hacktics

Hello everyone.
I searched the forum but I didn't quite find what I was looking to be answered.

I would like to know or learn some tips on custom portrait making.
I am in no way a digital artist, and my graphics editing is very rudimentary.

I am developing a Dungeons and Dragons campaign, and I wanted to create portraits to go along with some NPCs, and if possible, my players' characters as well. My laptop's OS is Linux's Ubuntu, so I only have Gimp on here. I tried to Frankenstein a few portraits by using free select to go over specific pixels and cut and paste them onto a different portrait, but I didn't have the follow up skills to clean up the rest of the image (i.e. matching hair colors). I've seen that Graphics Gale might be the way to go, and I have it installed on my Windows 10 PC. If I wanted to focus on portrait editing only, would I be okay using the ripped .JPG files from the game, or would I need to use the .BMP files and just screenshot/export the portrait?

I was also curious about making the portraits more HD, but I'm not sure how to go about that. I know the original portraits are 32px wide and 48px tall, and I believe allows up to 15 colors, (16 if you don't need the transparent black background?). I saw that there might have been another software to use to make the images more HD, but I just wasn't sure about any of this. I'm fairly new, but I've always loved the art of FFT and I guess I want to translate that passion into a new passion.

So I guess the TL;DR is where can a noob start with making original or custom portraits? A lot of people on here seem to have been in this craft for a long time and your work is beautiful and fits right in with the original art. I would like assistance so I too may work at this.

Currently there's no way to use higher-resolution portraits in the game.

When starting a new portrait, it's a good idea to refer to the originals to get an idea of what is consistent between them so that yours can follow suit. You can view/export them in Shishi, or you can look at this portrait reference sheet (also includes some great custom portraits by others)

Some general guidelines - try to use 'FFT' colors (e.g. not too vibrant or saturated), the top of the head normally cuts off or meets at the top of the portrait, and make the most out of the 15/16 colors you have. Also, portraits will appear stretched in-game (I use FFTEVGRP to preview how they'll look in-game). Flip your portrait from time to time to see mistakes you may not have noticed before

Portraits of decent quality take time for most people. But really, as long as you're making an effort to stay consistent with the vanilla portraits, they should turn out fine

I am not looking to import the images back into the game at this time. I am only looking to make portraits either by combining parts of different portraits, or by drawing in my own stuff. I do want to follow the 'FFT' color scheme and I don't have an interest in making things too vibrant or saturated. I just wasn't sure if I needed the sprite bmp in order to work on the portrait's colors, or if I could just use the .jpg of the portrait in graphics gale to alter the color palette. I will look into Shishi as well, but like I said, I'm currently trying to experiment with customizing to make "original" characters.

I don't have any issues spending hours or days working on stuff. I care about quality and I do my best to pay attention to detail. Flipping the portrait is good advice, and I'll remember that when I go to work on it. I'm currently just bad with using colors for shadows and highlights, and not quite sure how to place colors/pixels for best results and optimal blending