Your main library may have copies. Also lots of University libraries hold copies - where do you live?
There are also a couple of books aswell that have various pictures especially the war time ones.
Also many Picture Post photographers were very well known such as Bert Hardy, Humphrey Spender, and, occasionally, Bill Brandt so you aunt may have been lucky enough to been in one of these fellas photos and therefore may find the picture in a book by them.
But perhaps your best bet is here:
http://creative.gettyimages.com/source/home/home.aspx
PP was started by Edward Hulton of Hulton Getty fame and all the images were transfered to the getty archive. You should find it here somewhere (although you may have to pay for a copy)