OK, I figured it out why I wasn't getting anywhere.
In the maze of DVD (and CD) formats, I didn't realize I was using a DVD+R disc. I had written a single file to one disc and then tried to burn a folder to the same disc not realizing DVD+Rs are only write once. So now I guess I have a 'coaster' with a single 'jpg' picture on it.
Inserting a second NEW disc, I did indeed succeed in burning a top folder containing many sub-folder each containing many 'jpg' pictures. ImgBurn did this quickly and I could read the disc just fine in another drive unit.
Just as a suggestion, for noobies like me, it would be nice if ImgBurn could advise the used that disc in the drive has already been written to once and that no further burns are possible. I guess the little 'Disc is not empty' message is supposed to be the hint for this.
Anyway, thanks for the suggestions so quickly. ImgBurn rocks!