How do I release a filechooser image?

local variable current_file = 0
Filechooser -> On changed
  - PhotoField -> Load image from filechooser.FileURLAt(current_file)
  - System -> Add 1 to current_file
[/code:3dyq52a7]

This way the user can load a new image to replace the previous one. But if I add 

[code:3dyq52a7]
Filechooser -> release file(filechooser.FileURLAt(current_file-1))
[/code:3dyq52a7]

to release the previous file the image doesn't load at all. What should I do to be able to replace the previous image with another and release the previous one?
Also I feel I shouldn't be adding urls to filechooser but replacing the url at (0). However, any attempt to do so results in the image not changing or not loading.