Basically, I downloaded some new themes where I don't really like the pic they supply. But the pic is optimized for the theme, so I may want to go back to it... BUT everytime I change the pic, I can never ever (on any OS - maybe I'm stupid) find the original theme pic that came with the theme. Can anyone help?
Can't. You'd have to get the images from the theme makers or somewhere else.
Shame you can't use Plazmic to "unlock" and pick apart themes. I have some changes that I want to make to the theme that I'm using now. I really like it, but there are just a few things that I would want to change.