First, in Finder, the icons are actually always thumbnails. If you keep all your images in the Pictures folder, or some other folder, then go to the top level folder containing pictures (i.e. Pictures) and choose View -> Show View Options. Then change the icon size until the thumbnails are sufficiently big. This will affect all sub-folders, so you should see thumbnails of adequate size for all your images. Not perfect, but....
Second, to import images from a folder into iPhoto, just drag the folder into the part of the iPhoto window that has the thumbnails, and it will import and show up in the last import segment.