There is no way to reset a VRC+ Profile picture in-client.
Adeon Writer
You can reset your Profile picture photo on the website, but there is no way to do this in the client itself.
By reset I mean return it to showing your currently worn avatar. The only way to do this in-client is to let your subscription run out.
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naqtn
I found a workaround in-client. Step 1. Take a new photo, 2. Set it as the profile picture, 3. Then delete it.
And there isn't a way to reset it on the website right now. I'm not sure, but maybe we had some updates.