Technically, if you post a picture from lemmy.ca to a different instance, the picture is stored on lemmy.ca's server, so yes, that would be against the rules, as that is what i'm trying to avoid.
I would say a general rule of thumb (as a lot of instances have similar rules, but they're probably enforced differently) that you try and follow the rules of both the instance you're posting from and the one your posting to. the community itself can have it's own set of rules too.
As to where the content itself is stored, technically, they're stored in both places. if you delete your account, i believe it gives you can option to delete all your content as well, and those deletes will be federated as well.
That being said, if one instance goes down, or an owner takes it down, or whatever, any content that has already been federated to other instances are already there, ie, stuff should still work, for the most part, with probably the exception being any images stored locally on the instance that is down, since images themselves don't federate across instances.
Also, a quick disclaimer, all of the above is all stuff that "i'm pretty sure on", haha, just from my understanding of how things work around here. If i'm wrong on anything, feel free to correct me!