f = discord.File("some_file_path", filename="image.png")
e = discord.Embed()
e.set_image(url="attachment://image.png")
await messagable.send(file=f, embed=e)
This allows you to use a local image located with the bots files, instead of uploading first and then specifying that uploaded image's url.
Just load all of the images into an array and use a random math function if you want to select a random image. If not, then name your pictures the name of the pokemon and grab that image. Shouldn't be difficult. Just use
.setImage('attachment://yourfilepath')
in your embed builder.