Click it and it will show the full size image
It's showing thumbnail sizes, to conserve space on a thread. This could be helpful if a thread has a lot of pictures in it. Also, right now, we have to limit the sizes of images so they fit in a post. If we use thumbnails so that people have to click to see the full size image, then we can allow larger images. I hope that didn't sound too confusing, lol.
I'll see if I can edit the "attached thumbnail" to indicate to click it.