It usually takes a few minutes for the pages to update.
Very rarely, but on occasion, Github Pages runs into a problem and doesn't upload the changes. You can check this by going to https://github.com/adafruit/circuitpython-org, going to the Settings tab, if you have access, and scrolling down to the Github Pages section to check for an error.
Submitting a new PR will cause it to repair itself. If you want to submit an empty Pull Request (no changes), you can first create an empty commit with the following command:
git commit --allow-empty -m "Trigger GHPages Rebuild"
You can change the message to be whatever is appropriate. After that, submit a new Pull Request.
You will want to check that the image specified in your markdown file matches the image name and does not include a path.
First try refreshing the page in case the data is cached in your browser. If that doesn't work, make sure all of the image filenames match.
If the new board you added is showing up and there is also an existing ID for your board, make sure to use that first. To find the ID, Right-click on the Unknown board and choose Inspect. Look for the data-id tag to get the board ID.