Now that your display is up and running, you can push new GIF URLs to your Pi from any web connected browser using the text block on Adafruit IO. Click on the text to change it, type or paste in the new URL, and press Return or Enter to send the new value.
Your Raspberry Pi should now display the new image. If not, you may have entered the URL improperly. Keep in mind the server will only accept URLs that end in a .gif, .png, or .jpg extension.
You can also push new URLs from any of the available Adafruit IO client libraries. Refer to the library documentation for more information about pushing values to a feed.