"There was a picture of Dewi Lingo in the CCTV camera."
Translation:Roedd llun o Dewi Lingo yn y camera CCTV.
There is a brilliant series on youtube explaining pronunciation.
Regarding CCTV, the pronunciation is correct as using the name of the Welsh letters but not as the letters are pronounced in a word which would be 'Cuh, Cuh, Tuh, Vuh' but since this is not a word then the first would be the way it could be pronounced in real life.
However it would be more likely in real life to use the English 'See See Tee Vee'.