Apparently the photos are selected from your library by some criteria that includes recentness, You can set one specific photo by bringing up the picture to being full screen, then bring up the app command bar( ctrl - Z , and selecting 'set as app tile'. Source: Tom's Hardware
As is there's no obvious way to select folders, so I'd guess the solution would be to store things you don't want turning up outside the library. Tweaking metadata to reduce interestingness might work as well, but thats an extraordinarily ugly solution, and one that would need further understanding of the criteria the app uses.
EDIT: Further investigation shows it uses your pictures library, and by adding a folder to your pictures library it shows up there. Likewise removing a library removes it from the 'playlist'. Quite simply the way to remove a folder from showing up on the photos tile is to remove it from the library, and adding a folder to the library adds it to the tile.
There does seem to be a certain amount of 'lag' between changes in the library structure, and photos catching on with it.