- Keeping the diamonds from going out of the blue boundary (bouncing back if they hit an invisible boundary object while still dropping)
Try using solid/platform (diamonds solid, blue wall platform). And make sure you activate the physics and 8-dir.
- Adding an object that when zapped: adds +3 seconds to timer OR slows down all diamonds (optional stuff to enhance the gameplay)
You can use the "timescale" system property to slow time down.
- Showing the other picture when selecting it from the menu (I couldn't not get this to work at all after multiple attempts; not sure why)
Maybe store the "other picture" in another frame and flip to that frame? This feature issue is vague.
- Ability to view the unlocked pictures in the title menu (need some ideas to get this working)
You will want to use an array and either local storage or a server-side solution to store the game state.
- Add to the score [increments of 10, 20, etc.] the diamond gives when zapped in a certain area (this explains the lines in the play area; need some ideas to get this working)
Make a function that adds score with an input of 1,2,3... etc. then just multiply by 10 in the function and add it to a global variable score.