unchanged are cards which are not changed at all, changed_deckbuilding are cards which were changed, but only for the purposes of deck construction (influence, minimum deck size, limit 1 per deck), and changed are all other cards. Inside each folder, there are .zip files with names ending in bleeds" and nonbleeds. The bleed images have a colored border extending a little further than the regular card, and should be used if you plan on having the cards printed.
Finally, there is a separate folder "post_dnd_unofficial" holding card images for Mumbad, Flashpoint and Red Sands. A first pass of Reboot changes were made for them, but we have decided not to add them to the Reboot cardpool proper, so can't vouch for their balance. Still, feel free to print and play with them if you'd like.
(Note that the images have quite high resolution in the hopes that printing will look nicer, but if you plan to use them for anything else (e.g. displaying them on a page), you should convert them to JPEG first.)