Traditionally or strictly, the preposition should be "into" because it involves the motion of the money from a position outside the piggy bank to a position inside it, rather than a static or unchanging position.
The classic "error" with these two prepositions is, "Don't throw paper towels in the toilet." Taken literally, it is a request not to throw paper towels around the rest room while standing in the toilet bowl. The people who put up these signs really mean, "Don't throw paper towels into the toilet."
Many users of English, including many native speakers, carelessly use "in" where "into" is appropriate. Those who do this in speech and informal writing might not do so when they are writing formally or carefully.