libbmp

The libbmp is a too easy to use library, there is only three functions, a function to load a bitmap, another one to write a bitmap, and a last one to free a loaded bitmap, you just have to declare a t_bmp object and passing its address to the read, write or free an image

The read function will load the bitmap into memory and permit you to manipulate the data very easily, in your t_bmp object, you'll have a variable width and a variable height, these will indicate you the size of your image, there is also a field named data, this is a 2D array of t_rgb structures, t_rgb contain the RGB pixel component and its color average

Do not forget that C encode its data differently than mathematics does, this isn't data[x][y] but data[y][x] to access the pixel at the coordinate (x, y)