$ ls -l 1479482882.21_quake.png
-rw-r--r-- 1 user user 1478038 Nov 18 16:28 1479482882.21_quake.png

$ file 1479482882.21_quake.png
1479482882.21_quake.png: PNG image data, 1683 x 947, 8-bit/color RGB, non-interlaced

$ grep -a -b -o IEND 1479482882.21_quake.png
1473685:IEND

So there are some data after the PNG
$ dd if=1479482882.21_quake.png of=raw.bin bs=1 skip=$((1473685+8))

Inspection of the data reveals a kind of palette at the end, but nibble-inverted
So let's swap nibbles
$ cat raw.bin |xxd -p|tr -d '\n'|rev|xxd -r -p > raw.bin2
$ file raw.bin2
raw.bin2: GIF image data, version 89a, 480 x 25

The flag is in the new image:
GH16{20_Years_Olds_Game...Still_Played_!}
(and now I realize the title was a clue... "ekauQ")