So, I've been screwing around with the Gen III Mystery Gift Tool that can be found here:
Or, more specifically, the e-Reader Berry editor section of the program. The program lets you extract Berry images from an existing Berry file, but I've been trying to figure out just what kind of file format the Berry pictures use. The program doesn't give me any file extensions to work with, and I haven't seen any documentation about it online, which says a lot about either my searching skills or how much of a crap people online care about the e-Reader Berries.
(Edit: According to the magical question mark in the program, the file format the sprites are exported in is supposed to be .gba. Who knew?)
If anyone has any documentation on the file format, as well as how to convert an BMP/PNG into a e-Reader-compatible picture, I would very much appreciate it!
Edit: So, I found the solution to my problem, but I'll document it here for posterity's sake.
Firstly, you'll need Nameless Sprite Editor, which can be downloaded here:
Afterwards, you need to extract the sprite from an existing ECB, and you need to give the sprite the .gba file extension.
Then, open the .gba file with NSE, click Navigate, then input the values:
Image offset: 20
Palette offset: 0
Width: 6
Height: 6
and then hit Open.
From there, you can edit the sprite of the Berry into whatever you want. As far as I know, if you want to copy and paste a normal image like a BMP or a PNG, you can't do that, so you'll have to remake the sprite pixel by pixel. Of course, I've barely messed around with the program, and if the default program doesn't feature something like that, NSE has a plugin system that might add that functionality.
After you're done, just save the .gba file you edited, and import it back into an ECB.
I realize that this is kinda on me for not noticing something so obvious, but the fact that I could find anything about this on Google made me think I'd hit a dead end unless I consulted a forum like this. As such, partially to own up to my mistake, and partially because I don't know where the delete button is yet, I'll leave this post up for any confused passer-bys who want to know how to modify data relating to a game almost 20 years old that used to be stored on cards.