Go to your DDNet data/gfx folder and copy the original game.png file.
Drag and drop your new custom .png files into the directory, ensuring they have the exact same names as the originals (e.g., game.png ). For Standalone Client Users Ddnet Texture Packs
The DDNet community is global and very active. Here are the best places to start your search: Go to your DDNet data/gfx folder and copy the original game