I use XnView to convert pictures. Free to download and use. I upload them to my PC via a bluetooth dongle then just open the pic and save it again. It gives you a choice of which format to save it as.
If you have a load to do it may be worth downloading a 30 day trial of Photoshop Elements and using the 'Process Multiple Files' option. You can make a load of other changes, such as batch renaming at the same time if required.
Lexus RC300h F-Sport, Yamaha XVS 1100, Toyota C-HR GR sport
Pretty sure you can do it using windows paint.
Don't forget a png may have different 'layers' (such as transparent' which mat not be reflected in the final jpg
His - Denim Blue A220 AMG Line Premium / Hers - Obsidian Black R172 SLK55
Google for FastStone Photo Resizer which will allow you to convert between many image formats, resize, crop, add text, watermark, etc. on an image by image basis or in batch mode.