There are several tools available to convert or resize images in bulk to other formats at once in one go. But I found this tool called PixResizer to be more effective and simple.
PixResizer can convert or resize images from one format to another in different dimension easily and quite fast. It can convert between all available formats, rotate image and not just that, it can provide several photo effects as well.
• Simple and free
• Can be used to create thumbnails
• Smart Ratio Calculation
• Batch conversion.
The tool is worth a try whether you are expert in photography or are just a novice.