There is nothing you can do to stop your pictures being 'stolen' in this way.
You can use javascript to stop 'right clicking' but the information can be gleaned from 'view page source' so is pointless.
Anything on a computer screen can be 'captured' and edited.
You can try threatening him/her with legal action under copyright laws, but s/he will know it's just 'talk'.
You can watermark your pictures, making it difficult for other people to use, but these too can be edited.
It's not safe posting photos on the net. Copying in this way is just one of the risks.