Avatar images in Recent Activity widget missing descriptive alt attributes.

### Summary

On the [Make WordPress Accessible blog](https://make.wordpress.org/accessibility/), the "Recent Activity" widget in the sidebar displays user avatar images. However, these images lack appropriate `alt` text.

### Issue

Currently, the `img` elements used for avatar thumbnails either:

- Have an empty `alt=""` attribute, or
- Are missing the `alt` attribute entirely

This negatively impacts accessibility for screen reader users who cannot determine the author of the recent activity without visual context.

### Screen Shot

Thank you...

Inspect the Recent Activity widget on the linked Make WordPress Accessible blog and locate where its avatar thumbnail img elements are rendered. Done means each avatar has descriptive alt text that identifies the author and no longer leaves screen reader users without that context.

Written by the indexing model from the issue text.