How create an icon-png file?

I am a bit confused. After uploading my object, I've got the note that my zip file doesn't contain an icon.png file ... even if I added an icon into my zip file. I created a small image (64x64) and saved it just as "testwall_icon.png". Was it correct or not? If not, what did I wrong?
How to create an icon.png file?

Thx for your help! ;)
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