Such an Issue also arises while converting Images from a Digital Camera. So Here is a Little useful app that let batch conversion be done with utter ease, minus all the fuss, that most costly apps come with. This little proggie is named Bulk Image Converter and is a freeware.
All you have to do is select the base folder, which has to be converted. Select the Input format of pictures, and the Output Format you wish to be the outcome in. Click on Start button, and within moments you are done. The app is a breeze, and converts the images pretty much good.
You can download it for free at SourceForge.net . here are the Related Links for Downloads: