As Chuck said, select "Challenges" from the main menu. You'll be taken to a page with four categories listed. Choose a category to explore and click on one of the links below it. (There are two links for each category because the challenge posting/submittal process changed a few months ago to use the gallery software. Since challenges never expire, the older challenges are still posted in the old format.)
The old challenge area is fairly self-explanatory. If you click on the "current" link, you are brought to a list of all of the challenges for that category. Click on one of them and you'll be brought to a page of thumbnails. Set the "Sort by" field near the upper left of the page to "Most Popular". This should
show the original image first (or near first). If for some reason you can't find the orginal image that way, set the "Sort by" field near the upper left of the page to "Date (newest first)". The "original" file will be the OLDEST, so you may need to go to the second or third page to find the original, depending on how many entries there have been.
There is also a tutorial on Challenge Navigation
that will help when you get to the point that you want to submit a challenge entry.