How to combine the two photos ?

I want to combine two photos.
1.My father alone with backgrond light green
2.My family photos with empty slot for my father beside mother
My father was passed away three months ago.
So I want to combine these two photos to ge a perfect family photos.
Can anyone tell me to use which software and teach me how to do it ?
Thank you in advanced.

I'm betting this is spam and even though I may be making an enemy I'm going to ban the user.