Description

The numbers... what do they mean?

Solution

When the image is opened, the following image is displayed.

Since { appears in the seventh character of the image, we can a find out that the flag format is picoctf{***}. This means that 16→P, 9→I, 3→C, 15→O, 3→C, 20→T, and 6→F are used.
And from this rule, we can assume that Letter Number Cypher (A1Z26), which converts numbers directly into alphabets, is used.

All you need to do is decode A1Z26 with Cyberchef or convert it manually to get the flag.
But don't forget to add {} and type the flag in uppercase.

Flag: PICOCTF{THENUMBERSMASON}