Sentences 1 though 4 describe the wedding picture, and they also tell us why you 'will always treasure' the picture, so sentence 4 is the topic.
1. My father looks handsome in his old-fasioned top hat and tails.
2. My mother is seated before him wearing a lacy gown
3. In the bacground is the old arched gate of my grandparent's garden.
4. I will always treasure my parent's wedding picture.