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
Hi. I use the "[node:title] | [site:name]" tokens for the main image of news articles, blog posts, and similar publications. It's a quick and practical automatic ALT.
How to Improve SEO Performance in Backdrop CMS
I understand how tokens work, but not so much their practical use in image alt/text (maybe caption?) text. Could you give an example or two?
How to Improve SEO Performance in Backdrop CMS
https://backdropcms.org/project/metatag https://backdropcms.org/project/xmlsitemap https://backdropcms.org/project/imagefield_tokens These are more powerful modules, in my opinion...
How to Improve SEO Performance in Backdrop CMS