Problems with images when converting from png to jpg

Hi

I've tried googling this problem but cant find a way to sort it.

My supplier has given me product images in png format.

unfortunately the images are huge so i'm trying to shrink them and convert to jpg.

I've downloaded a few programs to convert/resize the files but the resulting jpg is not the same as the png image (the background is white on png but you can see the original background in the jpg).

I have discovered that using software (came free with either my digital camera or printer) if I resize the png image and then use any of the programs i downloaded to convert the image, then the jpg is the same as the png.

I have over 5000 images so i really dont want to have to do each one manually.

I just bought this stuff:-

coolutils.com/Online/Image-Converter

and successfully converted to jpg over 12,000 gifs and png that had black-defaulted transparent backgrounds.

For a few quid it's a brilliant program.