General question from someone just getting started with Elgg

I've been tasked with making a plugin that extends the functionality of tidypic. When you click on "add photo" a regular upload page, where you are able to load the images into an generic upload folder. And frankly I need some help.

First of all, I know how to extend the view of the core files into a plugin in order to change them, but how do I do this with another plugin's files? I've been playing with it a litle bit but with no avail.

I also need a little help understanding how the databse works for ELGG I can see from the database that when you upload an image there are several tables in the db that are updated. like elgg_river, elgg_objects_entity, elgg_metastrings, elgg_metadata, elgg_entity_subtypes, elgg_entities. Is there a way that ELGG handles this? Since the names of the tables can be different (if you use something diferent during install). I was to make something that is usable, since if I make a function that just injects the data into my tables, there is no guarante it will work for everyone using the plugin.

Thank you very much for your time and your patience! I hope you guys can help me understand hw ELGG works a little better.