If you have a scenario where Dynamic Media (also known as scene7) is configuredwith AEM, then you no need to worry, as soon as image in uploaded in AEM, it gets synced to Scene7 server.
Now as per you question some external assets needs to be synced to scene7.
1) If less assets are there, it can be manually uploaded in scene7 server.
2) If more assets are there and needs to be synced from third party system to scene7 then yes API is the only option left, and YES it do sends image as binary data only along with assets metadata details.
To know how to integrate AEM with Dynamic Media click here.