You could store them on a third site for static content, then both sites would link to that content. That can give you some benefits if you really need scalability as well.
However, you may also store it in a shared database, or just a shared file share, that you have encapsulated logic that each sites that needs access to them uses.
If you dont want to both with the storage there are many "cloud" storage servers that will host all your images for you. (like Amazon S3, Smugmug, flickr).
But then you will have to build the logic in your app to upload the submitted images to your 3rd party storage provider.