I've incorporated the following suggestions:

- Order of precedence:
  + #+attr_org overrides #+attr_html and #+attr_latex
  + `:center t' overrides `:align ...'

- Update doc/org-manual.org under the images section.

- Add a checker for `:align nil' to org-lint.  `:align nil' is not
  supported.

- Unspecified behaviors:
  + The behavior of `:center nil` is undefined
  + #+attr_html vs #+attr_latex

- Include a commit message in the patch.  The included patch is based on
  the main branch.

Karthik