Raspberry Pi has scrot command installed from first and you can take snapshots by hitting the PrintScreen key.
But you will get a black window if you try to take a snapshot of Minecraft screen, very sadly.
raspi2png is very good to make it. But unfortunately, you cannot install it by apt-get. Let's install it and assign the PrintScreen key to launch it. Look at here.
You can pass the Step 1 if you already have raspi2png, please check it. Type raspi2 in the black window of LXTerminal, then press the tab key. It should be auto-completed to raspi2png if you have the command installed.
Step 1. Install the raspi2png
Copy the 6 lines below and paste them into LXTerminal, then press enter key twice to finish. Downloading, expanding, compiling, and relocation would be automatically done.
Step 2. Assign PrintScreen key to launch the command
Same as Step1, copy the 6 lines below and paste them into LXTerminal, then press enter key twice to finish. It creates a script file to launch the raspi2png and assign PrintScreen key to launch the script file instead of scrot command. Snapshots will be stored in the Pictures folder with names like snapshot-20180114-193128.png. It's snapshot-YearMonthDate-HourMinuteSecond.png.