I think you save the screen shot and make a dummy account on psn. Then you send the screenshot to the dummy account and have this dummy account signed into the playstation app on mobile devices.
Then you go to your messages on the playstation app and apparently there is the picture in it's full uncompressed glory which you can just save directly to your phone.
I think that's right? lol.
When you upload to facebook or twitter it's compressed and you don't want that.
I don't think you need to log in to your dummy account in the app, you can just go to your messages in your normal account and see your sent messages and pull the pic from there.