I had a bunch of png images in several subfolders which I wanted to convert into jpg.
I used the command line tool ImageMagick for Windows, which is free and can be downloaded here:
Download @ ImageMagick
This command will convert all images in the folder and its subfolders from png to jpg with quality 65. The converted files will get the same filename but with jpg extension:
for /R %f in (*.png) do ( convert -quality 65 "%f" "%~npf.jpg" )
convert is the ImageMagick command running, for /R is used for looping recursively.
Use this command when done to remove all the source png files:
del *.png /s
This will delete all png files in this folder and all subfolders.