Its easy to obtain a visa on arrival:

It saves time and hassle at the airport if you download the application form from a website such as:

the Kenya Immigration Department 

the Kenya High Commission UK  (at the moment the copy here is a rather poor scan)

the Kenya High Commission Canada

the Kenya High Commission India 

and complete it at your leisure at home. On arrival hand it with your passport and fee (in US Dollars, GB Pounds, Euros or Swiss Francs) to the immigration official. No photo required, they photograph you and most of the time they also take your fingerprints electronically at the desk. They put a full page visa sticker in your passport and stamp it.

Fees current January 2014:

Single entry visa $50 £30 40€

Transit Visa $20 £10 15€

(Multiple entry visas, valid 1 Year, cannot be obtained on arrival, they can only be obtained from Embassies or High Commissions)