How to upload iphone photos onto my computer in JPEG format?

I uploaded hundreds of pictures onto my laptop and tried to crop/ edit some photos but I can't even edit them because "they are in PNG format." In order to change the format I have to open each individual photo onto Paint and "save as" into JPEG format. How can I just upload the photos from my iphone automatically into JPEG format? Or is there a quicker way to change the format, rather then doing it one by one?

Open a Free PictureTrail account then upload your photos to its album feature. Picture Trail only accepts JPG format and will automatically convert PNG and GIF files into Jpeg format.

http://www.picturetrail.com