Another PNG this time with a black and white squares pattern.

Again a contrast and brightness trick.

Modified black and white image revealing ASCII characters

This time we encounter a strange pattern. If we think of the pure black parts as 0 and the black and white ones as 1 we get some binary code for each line.

BinHexASCII
010101000x54T
010010000x48H
010001010x45E
010000110x43C
010011110x4fO
010001000x44D
010001010x45E
010100100x52R

and it repeats itself.

-> flag{THECODER}