A significant number of extensions and skins on MediaWiki.org do not have a screenshot in their infobox.
- https://www.mediawiki.org/wiki/Category:MediaWiki_skins_without_a_screenshot
- https://www.mediawiki.org/wiki/Category:MediaWiki_extensions_without_a_screenshot
Pick an extension or skin from those categories to take screenshots of. It is ideal if you can take the screenshot on a vanilla instance of MediaWiki (using MediaWiki-Vagrant for example), but some extensions may be difficult to setup and configure, for those it's okay if you take a screenshot from a MediaWiki installation on the internet (WikiApiary can be helpful in finding sites with the extension installed). Make sure the screenshots do not contain any parts of your browser or OS, and just focus on the extension's functionality. See also Help:Screenshots.
Next, upload the images to Wikimedia Commons. Make sure you use the correct licensing templates, {{MediaWiki screenshot}} for vanilla MediaWiki, or {{Wikimedia-screenshot}} for Wikimedia sites. The image should be added to the category Category:Screenshots of MediaWiki extensions.
Finally, add the image to the extension/skin page on mediawiki.org by reaching that page, clicking "edit" or "edit source", editing the template and adding the |image= parameter.