Analysis with [Steganabara](https://github.com/quangntenemy/Steganabara) or any image editing tool with similar features shows that the suspicious black lines are on rows 127, 137, 147 and so on.
![Steganabara](https://raw.githubusercontent.com/CTF-STeam/ctf-writeups/master/2020/nactf/Static/static-steganabara.png)
Put them together and we get the flag: [static_sol.py](https://github.com/CTF-STeam/ctf-writeups/blob/master/2020/nactf/Static/static_sol.py)
![Flag](https://raw.githubusercontent.com/CTF-STeam/ctf-writeups/master/2020/nactf/Static/steganabara_zoomed.png)