So that's the problem. You must resize every tile that you're going to use. However, there're some tilesets pre-designed for Essentials, with their correct sizes. Anyways, you can't just download every image and add it. There's some work that you have to do resizing and/or cutting squares to place them in a tileset. You can use an old tileset to see how it's made, and the red squares with a red cross help a lot to see the limits of every tile. You can put or replace there everything that you want, but never changing its width (the thing of 8 squares of 32x32 pixels). Even, in Google you can search for "tilset bases for Pokémon Essentials" and you'll find some images that you can use as a guide.