Someone

void set_image_alpha(Image *image, unsigned char alpha) {
        int i, size;
        Color *pixel;
        unsigned int alpha_mod;
        if(!image || !image->data)
                return;
        size = image->textureWidth * image->textureHeight;
        pixel = image->data;
        if(alpha > 100) alpha = 100;
        alpha_mod = ((alpha * 255) / 100) << 24;
        for(i = 0; i <= size; i++) {
                *pixel = (*pixel & 0x00ffffff) | alpha_mod;
                pixel++;
        }
}