Change AI pngs for better distinction

[Chewbakka-Wakka]

I kept finding when mixing AI difficulties that the image distinction is not so clear, mainly between medium and hard.

This also adds back the original rabbit to be used for a supereasy placeholder (in prep for when it is ready to include matching to cam.)

It is quite hard to create an effective 24x24 png file to visually keep each difficulty setting clear, so it was a unique challenge and I am not going to admit how many hours it took to get it right.

As shown.
2 wheels, small tank - easy
3 wheels, medium tank - medium
4 wheels, big tank - hard

[Chewbakka-Wakka]

In terms of file names, I'd also rather change them all to group them up i.e.
image_easy -> image_bot_difficulty_easy
image_medium -> image_bot_difficulty_medium

[MaNGusT-]

Basic idea is good, design raises questions.

[Chewbakka-Wakka]

In fairness, I am aware you've done much work on the artwork side of things, no doubt.
By all means, these proposed pngs can be modified.