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WesleyFG didn't directly insert the graphics of the DS games into Pokemon Platinum. Pokemon games on the DS are in 3D and utilise models and textures in the overworld rather than 2D images like in the GBA games. WesleyFG took inspiration from a lot of the later games and redrew a lot of their 3D graphics as sprites rather than than directly ripping them and inserting them in Light Platinum.
There's a whole bunch of tutorials in the Tool, Tutorials & Resources section so I suggest browsing it to learn how to insert the various types of graphics used in third gen games (tiles, Pokemon sprites, overworlds etc.).