There are some software packages that I use daily to examine and resize images for academic research, paper-writing and photography. These are completely free of charge.
FastStone Image Resizer is excellent for batch resizing, cropping or conversion of images. This is freeware GUI-based software that is useful if you want to quickly do resizing without writing a script.
For script-based work, ImageMagick is useful for batch file conversions, but a thorough Google search is often required to find working examples. FFMPEG is similar to ImageMagick, but operates on video files. These two command-line based utilities are FOSS (Free and Open Source) software.