Description of the need
If you ever set the Default picture on admin/config/people/settings, then you could have an impression the settings in the next section for Picture display style would apply to the default picture.

Unfortunately, it's not true as the HTML-code always shows:
<img src="http://in.docksal.site/files/pictures/default_image.png" alt="Backdrop CMS's picture" title="Backdrop CMS's picture">
for the default picture. However, if a user uploads his/her own picture, then the image style kicks in:
<img src="http://in.docksal.site/files/styles/thumbnail/public/pictures/picture-1-1614678151.png" alt="Backdrop CMS's picture" title="Backdrop CMS's picture">
Proposed solution
I don't see any reasons for the style selected in Picture display style section not to be applied for also the default picture. If you, guys, support this feature request, then I could offer a respective PR.
Recent comments
I recently ran into the same error: Exception: The configuration directory in settings.php is specified as 'files/config_[some-random-CMI-ID-here]/active', but this...
Live deployment Configuration Directory error
https://github.com/shabib-m/tailwindcss/wiki
Free Backdrop Theme built on Tailwind CSS v3 (Based on Basis) - Seeking Feedback & Ideas!
Admin Edit - note the image above is a link to the YouTube video.
Free Backdrop Theme built on Tailwind CSS v3 (Based on Basis) - Seeking Feedback & Ideas!