Update: The episode is from the special "Wishology: The Exciting Middle Part"
Here's another show that just loves embarrassing it's main character in his briefs. All of the screenshots I've found for this show have been created by other people and it's been a long time since I've watched this show; so I'm afraid I don't know the episode for this and other Fairly Oddparents screencaps. If you never watched the show however, pick a random episode and chances are high you'll see Timmy Turner in his skivvies.
Update: The episode is called "Chin Up"