Here's the default profile icon: https://ssl.gstatic.com/s2/profiles/images/silhouette28.png You can change "28" to any size you want. Just download that, then upload it as your profile image :)

Only even numbers though. Interesting.

I did just that as Google kept replacing my youtube profile image with a random snapshot from a video I once uploaded. A blank white .png did the job.

I wrote for a music ezine sometimes, and once I interviewed a artist and asked him to say hello to our readers on camera. Since the ezine had no YouTube channel back then, and we were in a hurry, I uploaded the video to my own channel.

YouTube must have thought that, since it was a short (~20sec) video with a person's face depicted prominently, the person had to be me and updated my profile image with a snapshot of this video that actually looked like an intentional profile image. Maybe it was just random, maybe they actually tried to select a fitting image.

Geez, this is a little invasive. Must be an additional "marketing feature" to coerce you to upload a profile picture. Who'd have known a userpic is so highly valued in the social data market.

Nope. I have a monochrome image. I don't receive any notification.

I use a picture of a painted, corrugated metal wall that a coder/photoblogger posted (as one of several offerings for mobile phone wallpaper). I've never gotten a notice like that.

I did this on LinkedIn, because they have a rating system for being able to set up a business account (and so they basically require an image). It works on Google+ as well.