How can I repair cracks at corners of tiled walls?
The area on top looks like it was sealed with white caulk.  You can re-seal this with white silicon caulk.  The other cracks look like it's grout.  Do you have any of the grout the contractor used?  IMHO, clean the old grout out and regrout. I've never had much luck getting new grout to stick to old.  You can also run caulk over it as well but this will show up.