1) The code, for now, runs locally. This is good. To avoid the possibility of the code being tampered with at a later day (for example, it could be modified to send copies of the image to a server), download the webpage and use the saved copy, not the live copy.
2) Do not use the blur functionality. For maximum privacy, this should be removed from the app entirely. There are _a lot_ of forensic methods to reverse blur techniques.
3) Be weary of other things in the photograph that might identify someone: reflections, shadows, so on.
4) Really a subset of 2 and 3, but be aware that blocking out faces is often times not sufficient to anonymise the subject in the photo. Identifying marks like tattoos, or even something as basic as the shoes they are wearing, can be used to identify the target.
Any examples? You can't reverse it if the data is gone.
That's the problem - the data you think is gone isn't gone. High frequencies are gone.... but you left all the low frequencies, didn't you? You can read a face from the low frequencies.
How about replace each face with a "this is not a person" AI generated face, then blur+mosaic. Or just a non-person face using a deepfake system that matches the facial expression?
Don't tell people what not to do. Figure out why they're doing it, and provide what they actually want while still achieving the goals (here: security).
Very coarse mosaic, add noise, then blur seems reasonably safe, and doesn't have to look like crap.