Start by going to the search engine (or elsewhere) and searching for the GIF you want to display as your Chrome wallpaper. Select one that is, preferably of good quality. Then open the GIF in the browser, right-click and choose to save it as.
Now that your GIF is saved, open a new tab in Chrome and click on the Customize button in the bottom right corner of the window.
If everything went as planned, the background of new empty tabs you open in Google Chrome should display the animated GIF you submitted.