1. The Content Plugin loads the main image. If you find it doesn't meet your requirements, we can easily add a new plugin feature along support. We are looking to add move power to the plugins now that image posting is a key feature.
2. Most of plugins repeat the same pattern. You can easily pick one of them (Eg weblinks) and re-write any of the examples.
If you need more advanced features, we can recommend a specific plugin as reference code. We have more than 30 examples... so we've covered already a wide scope
Also, there's an API if you don't want to write a plugin and just insert a Manual Request in the queue. You have a cronjob to be called from the command line. So, it almost there to implement an AutoTweetNG command to be execute from an external source.
Please, let us know if you have any further question.
OK, and why don't you use Joomla's native option for the teaser/fulltext image instead?
At least you should think of providing the opportunity to so for people (like myself) who stick on clean and native Joomla solutions.