extra content fields for entries?

I’m trying to create an archive page that just displays thumbnails for each entry.
I presume somehow I need to create a new content field in my entry page, where I can place my thumbnail image and then call on that with a post query tag somehow. Any ideas how to create extra fields in WP? or what the tag would be to call on a specific field? do I ned to use a plug-in?

(I’m considering going back to MT or textpattern as it seems to be far trickier to get custom things working in WP. Anyone else find this?)