No. The previews/thumbnails are generated on the fly if you browse a folder or the gallery, but this process is rather slow. Especially if you have folders with a lot of pictures in it on a rather slow device like a Raspberry Pi. Threfore you may want to pregenerate the thumbnails.
I saw photos which weren’t generated yet, but then they took quite long time to show so I thought they just show original resolution but actually they were being generated so it took a bit time to show!