Our game doesn't have much to it yet. This is an advantage for us at this stage. We can experiment by changing the sprites and sounds.
The clown sprite is a picture that is 32 pixels wide and 32 pixels tall. You could replace it with a similar sized picture or design your own.
You can use most image editing packages to make sprites. Paint is more than good enough for the purpose.
Draw a sprite around the same size and try that out in your game. Remember that changing the size of the clown will make it easier or harder to click on.
To set Paint up to draw a 32x32 graphic, click on Image, Attributes on the main menu and fill in the boxes appropriately.
Sprites are transparent if the transparent box is checked when you add the sprite to the game. The colour which will be transparent is the colour of the pixel in the bottom left corner of the image.
The wall is also 32x32 pixels in size. Changing the size of the wall doesn't make much sense but it could be much more interesting.
There are only 3 sounds. The background music is a MIDI file - these are quite small files and make the kind of plinky noises you find in games.
You could record some sounds using Audacity or other suitable software. How about if the clown said 'Ouch' in your voice when it was clicked?