I just read the png spec, and came up with this as a minimal, zlib based png encoding of a truecolour image with 1-4 channels

pngencode.cpp

// by alex evans, 2011.  released into the public domain.
// based on a first ever reading of the png spec, it occurs to me that a minimal png encoder should be quite simple.
// this is a first stab - may be buggy! the only external dependency is zlib and some basic typedefs (u32, u8)
//
// more context at http://altdevblogaday.org/2011/04/06/a-smaller-jpg-encoder/.
//
// follow me on twitter @mmalex http://twitter.com/mmalex
//
u32 crc_table[256]={0};
void make_crc_table(void)
{
	for (int n = 0; n < 256; n++) {
		u32 c = n;
		for (int k = 0; k < 8; k++) if (c & 1) c = 0xedb88320L ^ (c >> 1); else c = c >> 1;
		crc_table[n] = c;
	}
}
u32 update_crc(u32 c, unsigned char *buf, int len)
{
	if (!crc_table[0]) make_crc_table();
	for (int n = 0; n < len; n++) c = crc_table[(c ^ buf[n]) & 0xff] ^ (c >> 8);
	return c;
}
u8* CompressPNG(void *img, int w, int h, int numchans, u32 &len_out)
{	int p=w*numchans;
	z_stream z={0}; deflateInit(&z,-1);
	u8 *zbuf=(u8*)malloc(53+(z.avail_out=deflateBound(&z,(1+p)*h))+1);
	u8 *scanline=zbuf+53+z.avail_out;
	z.next_out=zbuf+41;
	
	for (int y=0;y<h;++y)
	{
		u8 *dst=scanline; *dst++ = 0;
		u8 *src=((u8*)img)+y*p;
		for (int x=0;x<p;++x) *dst++ = *src++;
		z.avail_in=1+p; z.next_in=scanline;
		int err = deflate(&z,(y==h-1) ? Z_FINISH : Z_NO_FLUSH);
	}
	len_out=z.next_out-zbuf-41;
	u8 pnghdr[41]={0x89,0x50,0x4e,0x47,0x0d,0x0a,0x1a,0x0a,0x00,0x00,0x00,0x0d,0x49,0x48,0x44,0x52,
				   0,0,w>>8,w,0,0,h>>8,h,8,"\0\0\04\02\06"[numchans],0,0,0,0,0,0,0,
				   len_out>>24,len_out>>16,len_out>>8,len_out,0x49,0x44,0x41,0x54};
	*(u32*)(pnghdr+29)=htonl(update_crc(0xffffffffL,pnghdr+12,17)^0xffffffffL);
	memcpy(zbuf,pnghdr,41); // header
	memcpy(z.next_out+4,"\x49\x45\x4e\x44\xae\x42\x60\x82",8); // footer
	*(u32*)z.next_out =htonl(update_crc(0xffffffffL, zbuf+41-4, len_out+4)^0xffffffffL);
	deflateEnd(&z);
	len_out+=53;
	return zbuf;
}
void PNGTest()
{
	u32 img[200][320];
	for (int y=0;y<200;++y) for (int x=0;x<320;++x) img[y][x]=x+(y<<8)+0xff000000;
	u32 blen; u8*zbuf = CompressPNG(img,16,16,4,blen);
	FILE *f=fopen("test.png","wb");
	fwrite(zbuf,1,blen,f);
	fclose(f);
	free(zbuf);
}