This can be done in so many ways. You can use any momentos from a trip or occasion. I thought it would be fun to use some vintage postcards from antique stores. One of my favorite ideas is to do this with a wedding picture, surrounding it with old black-and-whites and sepia wedding photos of couples from both sides of the families (His and Her great-grandparents, grandparents, parents). See example I compiled below...
Inspired? All I did was layout the postcards and arrange them, then paste them right to the existing mat. If you have a poster-size frame you can just apply them straight to a piece of poster board, and do the same with the center photo (not technically a mat but that is what I used!). My frame is from IKEA.