actually there are already edited BW sprites sized for GBA in the art gallery section. but I will just link his website where you can download it directly from.
Now about inserting them. the best way would be through NSE, you might need to download a plugin for it but its by far the easiest way to insert/replace sprites. btw there are two NSEs, classic and 2.0, i would suggest 2.0 as it can do way more.