Access to, and usage of previously uploaded image files?

I'm pretty new here, so hope it's just me, but:

How do I get access to the previous uploaded files - the ones listed as attached?

Say I uploaded 3 files but only inserted 1, but later want to use one of the others.. how do I do that?

Also if I mistakenly inserted an image as "full size" but want it as "thumbnail". How do I change that?