Am I to assume this is for the Uncharted Waters: New Horizons game and the SNES iteration at that?
In most of ROM hacking, especially on consoles before this last go around, we rarely ever see encrypted anything unless it is for saves or things that trouble cheat makers. Compression, various types of generated graphics (palette swapping, overlays, things stitched together from multiple tiles... you say you are a pixel artist so I assume you have encountered all this before).
From what you have said it sounds like we need to take it from the top as far as graphics on consoles go. Starting simply there are two concepts
Tiles are much like they sound like and are small usually squares or rectangles that house data. Every console has their own way of doing it, sometimes they are the same between consoles but usually they are not and thus the methods are named for the console they come from.
Palettes are the information that makes up the colours used. They can also be used for various types of animation but that can be covered later. Though a game console might boast support for however many hundreds of thousands of colours in practice you will rarely have more than a few hundred (and often something like 16) for a given frame. This also means you have to take it easy on or even avoid the likes of gradients, shading and other such things if you are used to more modern styles of pixel art.
Now I said they are usually square but you can have custom sizes on a lot of consoles, on older ones most will probably see it in text (and I hope for your sake you do not have to deal with an every tile/character is custom size) but it is not exclusive to it. Setting the wrong size can very quickly turn things into "noise".
I will cut it off here for now as going too much further will probably just be me typing out another version of "intro to hacking" which is covered more extensively elsewhere.