There are any number of free image converters for Windows
Image converter is provided by Microsoft so you can be sure it is safe to use - other free versions are available online. (Please let us know if this link is broken).
Download, install and open the program - you will see this screen where you can determine the preferred output type and where you want to save them:
Click the Add Images button and navigate to the folder containing your images - select the ones you want to convert:
Click Select then tick the ones you want to finally convert:
If successful you will see the following message and your files have been converted:
