Identify the hidden meaning behind this picture?

Can you find the missing letter in the sequence?
S - T - I - L - ? - T - F - Y - C

Decode this picture on T-Shirt and tell what it says ?

They came out at night, without being called.
They lost at day, without being stolen.
Who are they?

What happened when the wheel was invented?

I came first on earth but second on the heaven.
I also came twice in a week but found just once in a year.
I stay away from months but you can find me in February.

What am I?

See the given image carefully. What you have to do is move the blue checkers in the position of the black checkers and vice versa. You are only allowed to move the checker to an adjacent empty space. Do it in the least possible moves.

No one wants to have it, but if one got it, he does not want to lose it. What is It?

Be active and find the killer in the picture Riddle below.

I have cities, but no houses. I have mountains, but no trees. I have water, but no fish. What am I?