The problem is, that you cannot just add the GIFs of the Pokemon into the Game by simply replacing the sprites in the /Battlers/ folder. RMXP and Essentials, they way they are laid out, load all the bitmaps into the Game's cache (BitmapCache.load_bitmap), same goes for the bitmaps that are used in the battle system, summary, or pretty much anywhere else. This means that the images get loaded directly into your RAM. Now, with the GIFs being rather large files, a lot of data gets loaded into your RAM. If you were to go to Task Manager, and the Processes tab, every time you'd load a GIF within the game, you would see the Working Memory of the game increasing. Eventually the game fills up with too much data and simply crashes. Which is why you need a different way of loading GIFs into the game. Everyone that just replaced the sprites with the GIFs has/will notice the same problem.