By the looks of it I'd assume you haven't saved the image as 8-bit, assuming you're using Paint. If you had it would have messed with your palette. I'd suggest either saving as 8-bit in paint (not recommended, it'll mess up the colours), using a program like TilesetPal to reduce the colours (will keep your colours but is tedious to do repeatedly) or download a program that'll keep the palette intact like GraphicsGale or (maybe?) Paint.NET.