Using the directory:
/images

will allow images to that have or not have the underscore separator in the filename, to indicate images that are additional images ...
fred.jpg
owns: fred123.jpg fredabc.jpg
or owns: fred_123.jpg fred_abc.jpg


Using a subdirectory:
/images/something

require the use of the underscore separator in the filename, to indicate images that are additional images ...
fred.jpg
owns: fred_123.jpg fred_abc.jpg