Is there a Mac app that will open all images within a folder, regardless of the folder depth?
Isn't this where you would want to utilize Smart Folders, and search for Images?

This tutorial gives an example, but I've outlined a basic overview below:


  
  Open New Smart Folder.
  
  
  
  
  
  Select the type of search (in this case, type in images, and choose    Kind:Images from the selections.)
  
  
  
  
  
  Locate the Save button and save the search.
  
  
  
  
  
  Choose a meaningful name for the search. Optionally, check the Add to Sidebar box.
  
  
  
  
  
  Note the presence of the saved Smart Folder search, presuming that you followed the above step.