Most likely that signals a hardware issue, which means that, if still under warranty, you'd have to send it back. If not, then a repair is in order.
Most likely it's a loose connector inside the phone, but it could also be a damaged display driver, a damaged LCD panel, or a software bug if it only happens with certain apps.
If you open the same image on different apps and it flickers, it's most likely hardware (certain colors might trigger it; check zooming in/out to see if it changes). If it only happens with one app, it might be a weird bug with said app.