Cartridge based consoles prior to the DS usually operated on an all in one principle, or incbin if your prefer C like terminology.
Hopefully you are also aware of palettes as it pertains to this sort of thing -- most console graphics are not bitmaps that include what colour each pixel is but instead a very boring exercise in paint by numbers with the key being the palette (which you can change to change colours, hence palette swap enemies and
https://www.todayifoundout.com/inde...-and-bushes-in-super-mario-bros-are-the-same/ ). Palettes are not always necessary to edit (if you know something is one colour then pick it from elsewhere in the image and use that) but it does help, you can snatch them from memory too and if not there are usually not very far from graphics.
To that end you are left with two main approaches
1) Get a tile editor. I like tiled2002, tileggd and crystaltile2 myself but most things should do the megadrive/genesis graphics formats (the ones there I have as they do GBA formats many others do not do).
https://www.romhacking.net/?page=ut...page=20&title=&desc=tile+editor&utilsearch=Go should have a bunch.
Open ROM, set likely settings (custom tiles are kind of thing a here but 8x8 is a good start, SMD or GEN will likely be the abbreviation), press down/page down a lot and scroll through the ROM until you find something interesting.
2) A bit more targetted. Various approaches here
i) Get to point in the game where the graphics are displayed. Grab data from VRAM (or whatever it is called on your system of choice -- the megadrive has some alternative names for some things) and search the ROM for that. Does not work if compressed or altered but alteration is rare and compression... was seen but not as common as the consoles a generation later on up to today.
ii) In the case of the GBA the compression can be keyed in on or watched happen from BIOS calls, for the megadrive/genesis it is likely to be a more custom affair (probably RLE or Huffman but have seen some fancier stuff over the years).
iii) Corruption. Much as it sounds you mess with parts of the ROM and whatever changes in the game is what that area deals with at some level.
iv) Tracing. The big boy method. Get to a point in the game where it is about to load (before a level, before going into a room, before pressing start on the title screen... it really does not matter). Here you use a special type of debugging emulator to set breakpoints. Here is a worked example for an old command line GBA emulator but the principle is the same whether modern PC game or commodore64
https://www.romhacking.net/documents/361/
I don't know what we are suggesting for megadrive debuggers these days and they used to be a bit more primitive than NES, GBA and DS stuff last time I went looking, sometimes you might have to go into tool assisted speedrun circles.
So more difficult but guaranteed to find what you are looking for it is in the game (and probably enough info to find out if something that was cut by developers -- find one boss setup and you can likely find the rest, hidden stuff tends to be next to known stuff, similarly end epic sidequest sword is probably a bit on from starting dagger).
v) Some kind of logging method.
https://fceux.com/web/help/CodeDataLogger.html is for the NES but you might get lucky and find an equivalent for the megadrive.
Alternatively if it is just ripping graphics then the same vram and palette viewers you are using to confirm the harder methods will also work with a print screen. Do remember you have cheats at your disposal as well (no need to lose battles, max stats is easy, max money is basic cheat searching
https://web.archive.org/web/20080309104350/http://etk.scener.org/?op=tutorial , walk through walls, inventory cheats will get those hidden items/end game items...) to speed this up.
