If the photos are in the backup in iTunes, the only way to get them back is to restore that iTunes backup onto the phone. However, that will overwrite the current contents of the phone and you will lose the phots that are already on the phone.
The only way to get all of your photos onto the phone would be to transfer the current photos on the phone to your computer, restore the backup in iTunes that has the other photos and then sync the photos that you transferred to your computer back onto the phone.
Bear in mind once again, that you will overwrite the contents on your phone if you restore the backup. You cannot selectively restore content from a backup.