Your challenge's landing page is your participants' first port of call; from here they register to take part or log in if they're previously registered. You can use this page to tell your participants about the challenge, set any rules, and link them to any external sites for more information or to your fundraising page. The section highlighted in the screenshot below is where your message will display.
(You can find out about uploading your own logo here.)
Here's how to write your own welcome message:
Open Your Admin Area
Log in to your admin area (your challenge URL with "/admin" at the end).
Open the Current Challenge's "Settings" Section
Enter Your Message
Type your welcome message into the "Description" field. You can add images and links to this text.
Scroll down to the bottom of the page and click "Update" to Save Your Changes