We know from the challenges code that the length of key must be 12 chars . (If its not 12 the function returns an empty message). So we know that the pic is a png image and it has been encoded with a key , length 12. Maybe xor ??
We know that the png header has a specific format so we try to bruteforce the first 12 bytes of our encrypted pic with every possible key till it matches with the byte from the png header. Here is the script I wrote