The picture shows two complete chickens, not two pieces of chicken. While chicken meat can be uncountable, chickens are small enough for the whole bird to be cooked at home. The whole bird is "a chicken" and countable.
If you were to eat two of these, you would "eat two roast chickens". (That is more than most people could manage in one meal, but a smaller bird, like a pigeon, or a very young rooster, might be possible) A piece of chicken would refer to a joint or cut of meat. For example you might buy "4 pieces of fried chicken: two drumsticks and two thighs". Other animals that are small enough to eat are also counted "I ate 3 deep fried prawns", but if they are very small may again be treated as uncountable "I ate a pot of shrimp"
"Chicken" can refer to either the male or female bird, but the meat you buy is almost exclusively from the female "hen", as males (rooster, cock) are identified as chicks and killed.