XnConvert (Version: 1.51), from the maker of the excellent image editor and viewer XnView, is a freeware multi-platform batch image converter for Windows, Mac and Linux.
Input or load file(s)/folder(s) and then if necessary filter the images to only work on a set of them.
Next is the Actions tab, choose modifications or conversions required and see a preview on the right hand side. The actions available via the Add Actions button are extensive and are Add mask, Automatic crop, Canvas resize, Change color depth, Clean metadata, Crop, Extract channel, ICC convert, IPTC/XMP, Mirror, Replace color, Resize, Rotate, Set DPI, Text, Vignetting, Watermark, Adjust, Align histogram, Automatic contrast, Automatic levels, Color balance, Conbright, Equalize, Exposure, Gammasat, HLS, Levels, Logarithmic lut, Negative, Normalize, Normalize ex, Posterize, Saturation, Sepia, Shadow highlight, Solarize, Swap component, Temperature. Available filters are Average, Blur, Edge detect, Emboss, Emboss more, Enhance detail, Enhance edges, Enhance focus, Focus restoration, Gaussian blur, Maximum, Median box, Median cross, Minimum, Reduce noise, Sharpen and Soften. Others included are Add noise, Bloom, Border, Border 3-D, Crystallize, De-interlace, Fantasy, Halftone, Lens, Mosaic, Oil painting, Oil painting 2, Old camera, Photo spread, Pointillize, Retro, Shear, Slice, Spread, Swirl, Symmetry, Tile and Waves.
The output section has settings for choosing the output folder, format and an extensive number of options for renaming the image files. Everything from date to folder to increments and metadata can be selected and configured. XnConvert can delete the originals, keep any folder structures and keep multi-page files as multi-page and open windows explorer after conversion is complete.
XnConvert is complete and very easy to use, much like XnView itself actions can be saved as a script for future re-use. Both 32-bit and 64-bit editions are available as are installer and portable versions.