How do i extract the colours from a 24-bit image so i can make a custom palette? All i know is that the following image has 157 unique colours...

TilesetPal is what you want. There's also Disguise's Tileset Resource in case you want to learn more about palettes. I would suggest creating separate images from the original based on color range and then reducing the colors for them individually. But that's not really needed if you don't want an extra professional touch and you aren't planning on making different palette versions of your tileset.