This is a valid privacy issue, and other fediverse projects like Mastodon already solve this. The problem is that by embedding an image, you can tell the client to make a network request to your server, revealing information such as your IP address and browser. The solution is to proxy media through your instance, which is presumably trusted. this hides your IP address and browser information. And as someone else mentioned here, a Content-Security-Policy can be used to ensure this attack isn’t possible in a browser.