Configuring ArcGIS Pro Pop-up with same Image for every feature?

You entered a link to a web page[1]* as the Source URL, not a direct link to the image[2].

  [1]: https://commons.wikimedia.org/wiki/Ship#/media/File:DANA_2004_ubt.jpeg  # Web page with image viewer
  [2]: https://upload.wikimedia.org/wikipedia/commons/9/9a/DANA_2004_ubt.jpeg  # Image direct link

* For info, the web page you linked to uses the WikiMedia Media Viewer:

What is Media Viewer?

Media Viewer is a new multimedia browser that aims to improve the viewing experience on Wikipedia and Wikimedia sister projects. It lets you view images in larger size, with useful information about their contents, authors and related metadata. It also offers a number of tools to share, download or embed media files.

The answer of @user2856 works well given that you need the entire layer in the table of contents to view the same image. However, if you want each individual geometry of the vector layer to view a different pop-up image you need to store the image link inside a text field in the attribute table as follows:

<img src="https://upload.wikimedia.org/wikipedia/commons/9/9a/DANA_2004_ubt.jpeg" style="width:100px;height:100px;">

Then call a pop-up as Text not Image:

and select the field that contains the link. In this example the field name is Link:

Now you can see different image for each geometry - polygon in this example.

The advantage of using this method is that you can control the size of the image to make it smaller or bigger using the style CSS included in the attribute table style="width:100px;height:100px;