CommunityCommons has a function Base64DecodeToFile to store Base64 encoded content in a FileDocument. Just make sure your entity is a specialization of System.Image
Woah! It worked!
I already passed by that Java action a lot of times but was hesitant to use it since I thought it was ONLY for decoding B64 to FileDocuments. Turns out it can also be used to decode to Image. Just need to set entity generalization to System.Image.
Thanks a lot!