Sed: replace a specific part

I need to replace a specific part of a link

for example:

http://sub.somedomain.com/s/be2b46b4cb28ed64fe59d191cb600117/2013/image.jpg

to

http://sub.somedomain.com/s/123123/2013/image.jpg

what I've tried:

echo "http://sub.somedomain.com/s/be2b46b4cb28ed64fe59d191cb600117/2013/image.jpg" | sed "s@/s/(.+?)/@123123@g"