You can technically, but you have to know a little about the image processor... there's not an easy "cut and paste" method yet.
If your gallery is, for example, http://www.trisweb.com/photos/etc/PB047612.JPG
http://www.trisweb.com/photos/etc/image/480/PB047612.JPG - 480px size.
http://www.trisweb.com/photos/etc/image/240/PB047612.JPG - 240px size.
http://www.trisweb.com/photos/etc/image/thumb/PB047612.JPG - thumb size.
Basically place "/image/[size]/" between the album name and the image filename in the URL.