If you need to convert many images in several folders replacing the original images with images of a different format, size, etc, IrfanView can do it.

1: Open IrfanView and press the b key to open the batch dialogue (File > Batch conversion/rename)

2: Check the work type (batch conversion, etc) / Select output format / Check “Use advanced options”

3: Open the advanced options and in the Miscellaneous section check: “Overwrite existing files” — or, if you create images of a new format, and you don’t need the original, check “Delete original files after conversion“. Check “Create subfolders in destination folder” (it doesn’t matter if the sub-folders already exist; this has to be checked. Press OK to close this dialogue.

4: Make sure that the “Output directory” is blank / Check “Include subdirectories”

5: Find the source folder. Press the “Add all” button to add this folder and its sub-folders to the “input files” list of images. Press the “Start Batch” button. That’s it!

* Check also this list with the best free image converters, resizers and watermark embedders. Enjoy!