run like: cat source.png dest.png | a.out >out.png

pal.c

#include <stdio.h>
#include <stdint.h>
#include <arpa/inet.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>

typedef uint32_t U32;
typedef uint8_t U8;
typedef uint16_t U16;

U8 temp[100000]; //hopefully no png chunks are larger than this!

U8* read2(ssize_t n) {
	U8* dest = temp;
	while (n) {
		ssize_t amt = read(0, dest, n);
		n -= amt;
		dest += amt;
	}
	return temp;
}
void write2(U8* buffer, ssize_t n) {
	while (n) {
		ssize_t amt = write(1, buffer, n);
		n -= amt;
		buffer += amt;
	}
}
U32 read32be(void) {
	read2(4);
	U32 res = *(U32*)temp; // uh idk if this is allowed... alignment and all
	return ntohl(res);
}
void write32be(U32 d) {
	d = htonl(d);
	write2((U8*)&d, 4);
}
void skip(ssize_t b) {
	read2(b);
}
void trans(ssize_t b) {
	read2(b);
	write2(temp, b);
}

int main(int argc, char** argv) {
	dprintf(2, "start\n");
	U8 buffer2[1000];
	U8* buffer;
	for (int i=0; i<2; i++) {
		if (i==1)
			trans(8);
		else
			skip(8);
		dprintf(2, "starting image %d\n", i+1);
		for (;;) {
			U32 len = read32be();
			U32 type = read32be();
			dprintf(2, "read chunk: %4.4s\n", (U8*)&type);
			if (i==0) { // source image
				if (type != 0x504C5445ul) // not PLTE chunk
					skip(len+4);
				else {
					buffer = read2(len+4);
					memcpy(buffer2, buffer, len+4);
				}
			} else { // dest image
				write32be(len);
				write32be(type);
				if (type != 0x504C5445ul) // not PLTE chunk
					trans(len+4);
				else {
					skip(len+4);
					write2(buffer2, len+4);
				}
			}
			if (type==0x49454E44ul) // IEND chunk
				break;
		}
	}
	dprintf(2, "done\n");
}