hey there,
i was just passing by and want to follow this post because its quite interesting.
Unfortunately i havnt done something like that yet, but i assume you want something like that:
During game the player can add a picture he has on his computer and import it somewhere inGame. For this he will have an UI with a button browse… to open up the FileDialog to select his image.
In order to get this working i think you would need to use the windows api (if you are using windows that is) to open that dialog.
sorry i am of no help here, but hopefully there will be one knowing that stuff 
best regards