It seems with all the people emulating games on the Steam Deck and how popular doing so has become, Nintendo has continued to take notice and fired off some DMCA requests to have images taken down (thanks GBAtemp).
To get started with: what is SteamGridDB? It's a website that hosts various images for various games, to help you add fancy artwork into Non-Steam games you add into your Steam Library. So instead of seeing Steam's default image for Non-Steam games, you get it all looking nice. Naturally, this means people will be uploading plenty of images they don't own, and often the official game images for various games on other platforms for when people are doing emulation.
Nintendo being clearly unhappy with this, fired off many DMCA requests and now you might spot several images across the likes of (each link an example of one removed): Pokémon Scarlet, Pokémon Violet, Splatoon 3, Xenoblade Chronicles 3, Super Mario Odyssey, and The Legend of Zelda: Breath of the Wild.
It's not just for emulation though, it has game pictures for tons of games that you can use as an alternative to the official images included with Steam.