I did not have any image files. I ran it in a VM (Virtual Machine). I have no idea why the image files were not written to disk.
I tried again. In the VM, I invoked a DOS shell (cmd.exe) and then executed the batch file from within that. I did this to observe possible error messages. There were two messages, missing *.jpg and missing *.ppm files. (Which are unimportant, see below.) My test .pdf was large (~30Mb, >700 images). As near as I can tell all images were extracted this time.
I also tried running the batch file without a VM but using a DOS command shell. Again all images seem to have been extracted. And the same two error messages were reported.
As a another test, no VM no DOS shell, just simply double clicking the batch file from Windows Explorer. Another success.
So to summarize, everything seems to be working fine. The results of my first attempt must have been an aberration.
The following will be unimportant to most people reading this thread, except those with a particular "techie" inquisitive nature or those having difficulties.
The error messages were a conundrum until I started digging. They are unimportant, since they are the result of the two erase commands in the .bat file. In addition to the .jpg image files, .pbm, .pgm and .ppm files are created, but not one-to-one with the .jpg files.
I went to the "superuser.com" web site (part of stack exchange) so I feel comfortable with the information I got there. The .ppm, .pbm and .pgm files have to do with image type within the .pdf. I'm not sure that I care enough to explore this further, but someone else might. One item of interest is the -l command line parameter, which gives an image by image extraction report. It might be helpful if an image you want from the .pdf doesn't extract.
Colin. Thanks for your help.