http://www.picresize.com/
https://f4rnham.github.io/BnS_pictures/
Find the picture you want, resize it in the first link to 378 x 620 and save it. Make sure that the picture you want to resize isn't smaller than that.
In the second link you need to first upload a valid picture of your character, and then the resized custom image. If it's successful it should open a new page with your new F2 picture. Save that in the BnS picture folder under CharacterShot. Then you should be able to find it in game in your Photogallery.