Looking at the new users tab there are several users that get served a blank profile picture.
Upon inspection the profile pictures link to a Error 400 page from Google.
When the URL is slightly adapted (Exclude
=s96-c=k-s328 ) the image is served.
The last part of this url is dynamic, as other values are also noticed instead of the
Credit mostly goes to Shadow.
Digging into it, it's possible to request different size, but only one at a time: