ImageManager lets you batch a whole bunch of files at once. Start by clicking on the Batch button in the toolbar of ImageManager. Then, select the directory you'd like to perform a batch on and disable Include subfolders if needed.
ImageManager currently has 3 types of batches; underscores to dashes, spaces to dashes and lowercase.
Before batching a whole bunch of files, you can analyze what ImageManager will do if you press the batch button. You can safely check if the batch and the selection is correct. This way you can prevent a bad outcome before executing it.
The batch button will execute the selected batch on the selected directory and if enabled, all child directories. The batch can be reversed so if anything goes wrong, you can restore the mistake.