Where to define static array related to an entity in symfony2 ?

I don't know if that's the best way but I used something similar to your code:

class Product
{
    protected static $category = array( 
        1 => 'animal.png',
        2 => 'blague.png',
        3 => 'devinette.png',
        // ...
        )
    );
}

Then you can add some functions in this class in order to get data from the array

    public function getCategoryImageFromIndex($a)
    {
        return self::$category[$a];
    }

    // if you have a getter getCategory() which returns the category of the Product
    public function getCategoryImage()
    {
        return self::$category[$this->getCategory()];
    }

Then you can call these functions from Twig:

{{ product.categoryImageFromIndex(1) }}

will display:

animal.png

And

{{ product.categoryImage }}

will display the corresponding image from the category.

I've always used a Twig extension function to access the static array.

For example, in my Order entity, I have something like this:

class Order
{
    const ORDER_STATUS_PENDING = 0;
    const ORDER_STATUS_AWAITING_PAYMENT = 1;
    const ORDER_STATUS_COMPLETE = 2;

    public static $ORDER_STATUS_DISPLAY = [
        self::ORDER_STATUS_PENDING => 'Pending',
        self::ORDER_STATUS_AWAITING_PAYMENT => 'Order placed',
        self::ORDER_STATUS_COMPLETE => 'Order completed',
    ];

then assuming you already have a registered Twig_Extension class, create a new filter function:

public function displayOrderStatus($orderStatus)
{
    return Order::$ORDER_STATUS_DISPLAY[$orderStatus];
}

Finally, use the filter in your Twig template:

{{ order.orderStatus|displayOrderStatus }}