why is evaluateCustomBlock.png executable?

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  1. None.
  2. Do NOT use root. That was just to hide my username from the image.
git clone https://github.com/jmoenig/Snap
cd Snap/help
ls
  1. evaluateCustomBlock.png is executable
  2. evaluateCustomBlock.png isn't executable

No.

Um, isn't it an expected behaviour for your computer to open the manual each time you execute the command .\evaluatecustomblock.png? Or have I got something wrong?

PNG files are not executable.
.\evaluateCustomBlock.png
gives

.evaluateCustomBlock.png: command not found

It might be the windows version though.
./evaluateCustomBlock.png
gives

./evaluateCustomBlock.png: line 1: $'\211PNG\r': command not found
./evaluateCustomBlock.png: line 2: $'\032': command not found
./evaluateCustomBlock.png: line 3: syntax error near unexpected token `)'
\7dQPe�T@D�%1˛�$o��uN�ra�hicConv��L���{�:_�n���z��������D�@��&��2Z���.Cak(�[�l���EF�qr��@�.�@$P�ٺ���8�b� DT0	�
                                                                                                                  ��_�'

On Windows, when I execute that command it opens the image on another window.

Windows doesn't have or know about linux executable file attribute bits. It does have file-extension based applications that it can launch if you try to run a file, which is what you saw. On a linux system, the executable bit would tell the shell to try to run the png directly, which would fail. You could experiment by changing the name of the file to xxx.exe and see what happens; I get a weird error that complains that it is an unsupported 16-bit application. Or, more directly equivalent to what spaceelephant saw, rename it to xxx.bat, where the cmd shell will treat it as a shell script and you'll get another weird error:
C:\Users\teleman\Documents\cs\snap\Snap-5.4.5\src>.\snap_logo_sm.bat

C:\Users\teleman\Documents\cs\snap\Snap-5.4.5\src>ëPNG
'ëPNG' is not recognized as an internal or external command,
operable program or batch file.

C:\Users\teleman\Documents\cs\snap\Snap-5.4.5\src>é
'é' is not recognized as an internal or external command,
operable program or batch file.

:thinking: