Augmenting sitecore's media library

Question

I am trying to find a way to augment sitecore's media library with media coming from an external source.

More specifically: I want to create a button within the media library that if you click on it, you see a list of images that are provided at run time (via an API) and you can select on one of them in order to add it on a sitecore web page or an image field.

PS: I am new to sitecore programming, so as much as possible detailed information is highly appreciated; but also guidance.