If you want to see all of the pictures that a user has submitted, across multiple Pic Challenges, the best place to look is on the user profile.  

Go to Users and then open the "Filters" section.  Search by the user's name and click on the user you are looking for.  

Once you are in the user's page, scroll down to the bottom of the user page and click on the "Pic Challenge Entries" tab.  From there you will be able to view all of the user's pic submissions.