RE: Converting a picture
You can open the png file in whatever image software you may have. It could be photoshop, GIMP, Microsoft Paint, or whatever software you may have installed that goes with your digital camera. When you open the file, click on file/save as. Then, save the file as a .jpeg or .jpg.
It should automatically convert that file into a new .jpg for you, and you'll also have the .png file saved in its original format as well.
Hope that helped