2019 MacBook Pro 15 inch: Screen flicker randomly
Reset NVRAM

If the screen flickers take effect particularly when viewing images while browsing the web, the best way around this might be resetting the NVRAM of your device. Resolving this issue involves rebooting you device. But because the NVRAM is a type of memory that saves data automatically, even when the device is forcefully switched off, this issue will not go away when you just reboot your device. 

To resolve this issue, switch off your Mac and then turn it back on, pressing down "Command" + "Option" + "P" + "R" keys until you hear some chimes or see the screen blinking twice during the restart. If that still doesn’t fix the flickering screen with macOS 10.14, reset the System Management Controller (SMC) as decribed below.

Reset The System Magement Controller (SMC)

This method applies only to portable Macs with an inbuilt battery like MacBook Air and MacBook Pro with Retina Display. Details of the steps for resetting the SMC of any other Mac device is also readily available on the web.

To reset the SMC, unplug the Mac's external display, and then turn off the Mac. Make sure the device is connected to the MagSafe power adapter. Press down “Shift” + “Control” + “Power” for a couple of seconds and then release them simultaneously. This resets the SMC of the MacBook Pro / Air instantly. Afterwards, press the power button to boot the Mac, and once the Mac is booted, plug back the display.