First you need to connect your phone to the computer and make sure the memory where the photos was stored is visible in windows.. be it phone memory or memory card..
Then download and install this software.. its free..
http://www.piriform.com/recuva
Then scan that memory with this software and it should be able to recover most of the deleted files..
One more thing, it only works with normally deleted files... if your files were securely deleted, then you won't be able to recover it..