I want to convert a MP4 sequence of 230 images back to their JPEG images - what is the best (easiest) way of achieving this?
Most of the MP4 converters already have a functions to save to JPG, but you will have to defined what type of JPG files you need or more exactly how the software should recognize the frames and export them to JPG. Basically, an application of this kind will jump from frame to frame and export it to JPEG while the manual way would be to manually pause the video, capture the screenshot and then save it.
You can use Free Video to JPG Converter for this operation: https://www.dvdvideosoft.com/products/dvd/Free-Video-to-JPG-Converter.htm