Preparing a monster image for implementation in the game.

Hello everyone. I've searched all over the internet, but I haven't found any clear, accurate, and helpful manuals or guides on how to add a monster to the game or, at least, how to implement custom images. For example, I have an image in BMP format extracted from a fantastic game. To clarify, I have some leading questions: How do I make the background blue? If I can't change the background, how can I use DefTool to position and create a shadow? Are there any thoroughly written guides on modding?