OS 10.5 - Thumbnails distort when pictures resized

I am new to Mac and have a question about thumbnails in Preview. When I first import a photo, the thumbnails work fine when using scrolling through ... though, if I modify the photo in anyway or change the size, the thumbnail in icon view works well, but if I want to use the scroll feature where they are all displayed like dominos on the top window area, any changed photo is now pixely and blurred. Any answers there? Also ... I am wanting to take all my folders in PC and import them to iPhoto, but am wondering if when I change the size of a photo to save space, does iPhoto keep them in the large size in the /originals/ folder? or can I pull them from that folder and resize them modifying the original to save space? Then of course, I have my pixely thumbnail problem as a result ....