A collage is thought to be a masterpiece made from things not used in that sense. So his full picture of his collage is a masterpiece or success and the haters seen in it were his and drive to create beauty, but several possible ways to see this:
His parking garage is full of nice cars that he got from being inspired to work his ass off by the people he hates who tried to shoot down his dreams.
His parking garage is full of the dead bodies of people he hates.
His parking garage is his subconscious mind and inside his mind is all his old memories of people he despised in his life since he was five years old.
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Lead is a composite of minerals, pebbles are also a composite of minerals. Here the “lead shower” means the barrage from his gun, but he’s also using its literal meaning to play off the pebble bit. Telling the person he’s cursing at to “stand the rain” is the polite way of asking them to stand in front of his gun.