Your best bet is to export content from another site first. At the moment, there is no direct import tool for services.
Once you have your content stored locally, the simplest way to upload files to Are.na is simply to drag them from your desktop:
Are.na supports a wide range of file types, but please note that there are limits on bulk uploads:
Free accounts are only allowed to upload 15 files at a time. Premium members can upload up to 100 files at a time. The maximum individual file size is 50mb.