Censoring Photos On Mac

Hey guys, not sure if this is the right place to post this but I'm trying to censor some stuff in a photo on mac. I put the photo into preview, converted it to PNG, selected the area, and hit delete. MacOS immediately put a white box over it and I then converted it to JPEG, not sure if this is what actually happens but I figured the lossy compression would benefit me and the part that I got rid …

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