Some pictures in the Image Gallery can only be unlocked by the "most determined ape." This will require completing every stage, collecting every KONG letter, and getting all the Puzzle pieces in Mirror Mode (see the "Mirror Mode" tip for details on how to unlock it).
Completing the main game will unlock a new area called the Golden Temple. Access to this area is barred until you succeed in collecting all eight Rare Orbs found across all the worlds.
Retrieve each Orb by discovering and completing the hidden temples in each world (see the "Unlock Hidden Temples" tip for details) OR purchase them from Cranky Kong's shops for 50 coins each. Gather all eight Orbs to unlock the Golden Temple!
Complete the Cloud levels (available after you reach the Golden Temple) to unlock the special Mirror Mode, which allows you play the mirror versions of all the game's levels. However, you will not have access to Diddy Kong as a partner AND your health is limited to only one heart for the duration of the mode.
You can unlock the music gallery (found under the Extras menu) for each of the worlds on the island by beating the boss for that particular world. It's worth noting that using Super Kong does not count as a valid victory for the purposes of unlocking the music tracks.
In order to reveal the secret temple hidden in each section of the map, gather all four of the K-O-N-G letters scattered across every level in each of the eight worlds. For example, find all the KONG letters in every Volcano level to reveal the hidden Volcano temple.