Here are steps to do so:

• Create an account on Wikipedia (if you haven't done already)
• Create a subpage of your user page and name it as common.js: https://en.wikipedia.org/wiki/Special:MyPage/common.js
• Write a javascript that will pick a random image from a category in Wikimedia Commons and choose it to replace all images in an article
  • See guide for writing user scripts on Wikipedia: https://en.wikipedia.org/wiki/Wikipedia:User_scripts
• Document your work on the task on the following page: https://www.mediawiki.org/wiki/ChickTech_High_School_Kickoff_2017/Tasks. Include task title, changes made, code snippet, and screenshot of output.
  • To embed an image on Wikipedia, first upload it to Wikimedia Commons, and then insert it on the article page. Learn more here: https://en.wikipedia.org/wiki/Help:Adding_image

Note: Remember that the effect caused by your code in the common.js file will only be visible to you.