I used to teach playgroup, and when we teach flashcards we really have to "flash" the cards 1 sec per card. You might wonder if they could really see it, but babies have nothing in mind, to them 1 sec is enough especially they have very short attention span, and they learn faster than us. If we flash them slowly, they will lose interest. Always make sure that when you show the pic you tell them what is the pic at the same time, don't be too slow otherwise they cannot relate the two.
If you want to teach Chinese and English, you better do them seperately. For example you do a bunch of cards with all different animals in English - dog, cat, panda, horse....After you finish the cards, you can use the same bunch of cards and repeat them in Chinese - 狗,猫,熊猫,马。。。。I would suggest to do the Chinese a few hours later to make it more interesting and less confusing.
By the way going to playgroup will help but since mostly is just once a week. To be able to learn English better especially you want your baby to learn the "accent", you better let your child listen to English more at home as well. If possible it is better if you bring them to friends who speaks only English. You don't have to force your child to stay still and listen to English, you just have to let them play and you can play disc with English songs or stories. Just by going to playgroup without practicing will not make them learn a lot, it's just a waste of money then. The playgroup I used to teach in has a CD that repeats the keywords and the songs that was taught in playgroup, that way even parents are not able to teach their children they still get to "practice" at home.
I speak Cantonese, English and Mandarin to my son, and he goes to playgroup to learn Mandarin and he has a Beijing friend to play with. He has no problem with his language skills. He might not develop his skills as fast but he can respond to all three language as well as sign language. So don't worry, two language is not too much, it's very easy to pick up and better to learn earlier than late.