How long is a contract?   
Contracts can be month-to-month or a period of months.  Longer contract, such as a year gives discounts. The 1st month's payment is due upon execution of agreement.

Is there any other cost?

Yes, there is a production cost that is due upon execution of an agreement.  Production cost may be waived with a full year agreement.

 When designing a picture, what should I think about?
 - use 7-9 words in industry standard for highest readability on a sign.
 - Use colors that are opposite on the color wheel.
 - Stay away from script
 - Don not put copy over the top of pictures.
 - Make sure the use of pictures is a good return on investment.  The picture must be bright and clearly show the intent  of the advertisement.
 - Bold is better.
 - Focus on one aspect of your business per advertisement.

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