test.c

#define STB_IMAGE_IMPLEMENTATION
#include "stb_image.h"

#define STB_IMAGE_WRITE_IMPLEMENTATION
#include "stb_image_write.h"

int main() {
	int x, y, n;

	unsigned char *data = stbi_load("testimage.jpg", &x, &y, &n, 3);
	stbi_write_png("out-image.png", x, y, 3, data, x*3);
	stbi_image_free(data);
	return 0;
}