Output 20x20 Image to Arduino serial monitor
The Arduino IDE's serial monitor is, to put it bluntly, awful.

Switch to a proper ANSI compatible serial terminal emulator (Tera Term, Putty, Minicom, whatever) and use ANSI control sequences to manipulate the screen.

The simplest ones to use are:

Serial.print("\e[2J"); // Clear the screen
Serial.print("\e[1;1H"); // Move the cursor to coordinate (1,1)


Use the first one to clear the screen in setup(), then use the second one at the start of the loop to move to the top-left corner of the screen (1,1 is top left - the coordinates are given as Y;X not X;Y and start from 1 not 0).

Print your 20 lines of 20 characters as either space (nothing) or your chosen "pixel" character.  The spaces overwrite the existing characters to help reduce flicker (clearing the screen every time would be just awful).

The way serial works (sending one character at a time sequentially) there is no such thing as "All at once". The above is the best you can get without writing your own program to interpret incoming serial data representing an image frame and displaying it.