Hi,
I don't know if this'll help anyone, but I was struggling like crazy with this, ... I couldn't see the thumbnails of my photos in Finder. Damn - tried everything including all the good advice I could see here - but nothing worked, just got bigger icons.
Then the magic. Turns out when your looking for thumbnails of your picture files (say jpg's) within a folder, there's a little "twistie" (triangle) by the word 'Preview' (under which all the file info appears, you know, Name, Kind Size etc.) ... all you do is click on the Twistie and down drops a thumbnail of your image, presto!
Not only that, but once you've done it, its set like that for all your photos and you can 'explore' all your files and photos very easily without getting tangled up in iPhoto!
Phew, - my faith in Mac & Finder is restored!
Cheers Jeremy, the Victor Meldrew of users!