Select all the cells that cover the picture and (i.e. A1:H10) and type the following formula in the Formula Bar:
Do not press ENTER. If you do, you will create only a single random number in the first cell.
Instead, Press CTRL-Enter. This will “blast” the formula to all the selected cells.
These random number will change whenever the sheet recalculates. We need the numbers to be static.
Highlight all the random numbers and click Copy. Without moving or clicking anywhere, select the lower part of the Paste button and click Paste Values.
This will replace all the RANDBETWEEN functions with static numbers.
Consider increasing the values of cells that will cover more sensitive portions of the picture (such as the faces) with higher values. This way, the mystery is held for a longer period.