Changing Art
Most of this is pretty straightforward enough, just find the image you want to replace and replace it lol.
For card artwork, all the card artwork is stored in the 2020.full.illust_j.jpg.zib folder. Use my ID list located here to find the ID's of cards you want to change, then just edit and replace the image. It should work even if you don't resize the image to match the size the game uses, 304x304, but I always resized them just to make sure as pendulum cards have different size images compared to every other card so that's something else to keep an eye on. Also take note of the 2020.full.illust_a.jpg.zib folder, this has the cards that are censored for people outside of Japan. So if you replace a card that is one of those 1012 censored ones, it won't show up unless you replace it in that 2nd folder. It also might be possible just to delete those images entirely but I haven't tried it and done editing cards for the moment.
Arenas- These are pretty straightforward to just change the arena image, but some have props or other images attached to them that might be able to be edited or create new ones perhaps.
dfymoo files For these files, they are actually attached to the image they are named after. So if you edit images or want to try different stuff there like making some parts larger (not sure how some of these will show up if you do that), you will need to edit these as well. I will use the chars image as an example. If you look at the chars image, you will see all the in duel avatars for everyone in the game. You can edit these how you see fit, but if you look into the chars.dfymoo file, you can open it in either hex or Notepad++, it will look like this-
That first section-
w 1486 h 1967
is the width and height of the image. In my old mod I made an entirely new image that was much larger and included lots more characters in it, so you would need to change the dimensions here of course if you want a different sized image that you recreate.
After that you will see this-
~ n AI_neutral s 1 1 128 128 ~ n AkiraZaizen_neutral s 789 1717 103 128 o 17 0 128 128
These are both the image that shows up in duel, and also the character that shows up on the duel selection screen. AI for instance, if you look at his image in an image editing program such as Photoshop or Gimp, that 1, 1 should be his coordinates for where his image is located, then would be 128x128 in size. Some characters do have weird sizes or are smaller, so be careful of that when switching certain ones.
Other files for Hud or UI elements also may have these dfymoo files attached as well, otherwise changing images is easy :)