Hi, ondoho and Shadow_7,
Thanks very much for your advice.
I finally discovered that the problem was with the second image file. Even though it appeared like a portrait just like the first image. A look at the second file's width and height in terms of pixels reveals something abnormal.
If it were a true portrait, pixel-wise the width should be smaller than the height. Actually its width is 563 pixels and
height 338 pixels for some strange scanning-related reason.
Since both original image files were obtained by scanning two pictures, I decided to re-scan them using
Simple Scan instead of the previous scanning program. Happily, the resulting new scans correctly yielded images with a smaller pixel width than height in both cases. When I re-ran montage, I obtained a picture with the two images appearing side by side properly as expected.
Code:
montage jdn1.jpg jdn2.jpg -geometry +0+0 jdn.jpg
Julianvb