Walkable area in point and click game.

I know this may sound dumb but i think im doing this all wrong,
i want to get a colisions from a black and white image of my background. should i be using a .png with the walkable area set to clear. i havent a clue with 2d stuff.

its a silly game for a compo so just something basic that works is ok.

I dont even know what comands to use i guess if im using an image its pixel perfect i need

Pete