Convert .png pics to .jpg

My grandson has a Galaxy S III (Android OS) When he sends me pics. I cannot open them in Picasa, Win. Live Photo Gallery or Win. Photo viewer. All I can see is the default windows picture.
Thanks for any help.

Tech Support Guy System Info Utility version 1.0.0.2
OS Version: Microsoft Windows 7 Home Premium, Service Pack 1, 64 bit
Processor: Intel(R) Core(TM) i7 CPU Q 720 @ 1.60GHz, Intel64 Family 6 Model 30 Stepping 5
Processor Count: 8
RAM: 4084 Mb
Graphics Card: ATI Mobility Radeon HD 4650, 1024 Mb
Hard Drives: C: Total - 461899 MB, Free - 386144 MB;
Motherboard: Dell Inc., 0J509P
Antivirus: Microsoft Security Essentials, Updated and Enabled