XnConvert is a highly recommendable image conversion tool that lets you convert your images in just a couple steps and with no hassle.
The program is compatible with over 500 formats, including the most popular ones (JPG, PNG, TIFF, GIF, BPM, and RAW) as well as many other formats found only in comprehensive programs like this one.
Before converting the images, you can decide if you want to insert a frame, signature, water mark, special effects, etc. You can apply these changes to batches of photos and still maintain each file's metadata. You can add colored backgrounds if you want to make your photos the same size without changing their proportions.
All in all, XnConvert provides a customizable process for converting images by the batch. You can even include a correction filter that gets applied to all the files.