Users will see the new option to post to one or both of these services if they're uploading directly through Yfrog.com. Once you've given either service authorization, Yfrog will then cross-post whatever you've uploaded, from anywhere--be it a desktop or mobile app.
The system worked well enough in my testing. Though, out of the three social networks, it's the hardest to find your photo back on MySpace, as it shows up as a rather nondescript link wherever your status updates are set to appear. Another small quibble is that if you're uploading from a third-party Yfrog publishing app, it will automatically post the photo to these services--that is, once you've authorized them through Yfrog.com. The only way to make sure an image you're uploading won't show up in these other places is to upload it directly through Yfrog. Hopefully an API update can change that.
Those who are serious about cross-posting something from one social network to another should check out a service like Ping.fm, Updating.me, or Tarpipe. All of these can be set to watch one feed and transmit them elsewhere. This can be a huge help if you don't feel like fiddling with options on each specific network.